I've been doing a bit of research on these questions and have some guesses to share..
Apple has stated that the iPhone will be in Asia in 2008. We know that EDGE won't work in Japan but 3G standard UMTS / HSDPA will. Japan is far and away Apple's largest market in Asia, the world's second largest economy and most likely part of the initial iPhone rollout there. Apple has, so far, been rolling out iPhones in six month intervals and the one year anniversary of the iPhone release is June 29th. All signs are pointing to mid 2008 - May-July-ish.
The EDGE-speed iPhone is accepted in the US because 3G is just being built out and frankly Americans don't know what they are missing. However, in Europe, the lack of 3G access is a much bigger issue. We know that the iPhone isn't meeting European sales expectations largely because of its lack of 3G data access (also because people are bringing them over from the US unlocked). If Apple thought it could coast by on EDGE in Europe, it was mistaken. Therefore, it needs to start selling the 3G version there ASAP.
Hardware-wise, Apple has been working on the 3G iPhone since before the EDGE version was released. Chips that run 3G speeds at EDGE energy use (the reason Jobs gave for using EDGE) become available in quantity in Q2 2008.
From September 18th:
Steve Jobs, Apple's CEO, has confirmed that there is a 3G iPhone in the works, although fell short of saying when we could expect to buy one.
Making the comments at the "Mum is no longer the word" press conference at the Regent Street Apple store in London, Steve Jobs, CEO of Apple said: "You can expect a 3G iPhone later next year".
The news comes as a 2G EDGE-enabled iPhone will be available in the UK on 9 November.
When asked why the current model didn't have 3G, Jobs blamed power issues saying that the 3G chipset would be too much of a drain on the unit's battery life which promises 8 hours of call time, but said that future models would have the technology.
"We are working on the next iPhone already, the one after that and the one after that", Jobs said.
Apple did, however, just release a new version of the EDGE iPhone (the 16GB variety) and needs to wait a few months before releasing a new version in order not to upset its customers.
Oh, and don't forget college graduation gift time. What better way to send that grad off into the world but with a shiny new 3G iPhone - you might even be able to spend that government-issued $600 tax refund on it.
So with this information, I would peg the 3G iPhone release at around a May-June time frame.
The price is still a mystery but we know Apple wasn't selling iPhones at the rate they wanted at the $599 price point. Currently, they sell the top of the line 16Gb EDGE iPhone at $499. Therefore, I think they will need to drop the prices of the current models to make room for the 3G version(s).
Perhaps they could sell an 8GB 3G version of the device for $499 - but people willing to shell out for a 3G model will likely want top of the line features - so 16 or 32Gb options are more likely. I think Apple drops the prices on the EDGE models by $100-200 and sells them simultaneously for a generation with the 3G equivalents.
Why would anyone buy an EDGE iPhone when a 3G model exists? Perhaps Apple's carrier partners will also have a tiered pricing system in place. One price for EDGE, one price for 3G. Obviously, people with 3G iPhones will be taxing the networks much more, so it would make sense. Expect to pay a premium for 3G iPhone service - or maybe they'll keep the current pricing scheme for 3G and lower the price on EDGE access?
I think $499 for 16GB 3G iPhone
UPDATE: (It looks like Apple will let AT&T and the other carriers subsidize the price of this iPhone. $199 is the new starting price)
A lot of new smartphones are coming out with serious specs, besides 3G, that make the iPhone's look feeble. The Broadcom BCM21551 3G System on a Chip includes a lot of these next generation goodies. It is rumored to be the next gen wireless chips in the iPhone.
Screen size. The three-inch 800x400 to 720x480 pixel screens that the Nokia N810 tablet, Toshiba Portege G900, and Sony Xperia X1 sport are much better for video than the current iPhone's 480x320 display. These high resolution displays are physically only slightly bigger than Apple's current display but offer over twice the pixel range. Unfortunately, I don't think it is likely that Apple will implement these displays during the next revision (although I really hope I am wrong on this). The iPhone OS is too closely tied with the resolution, and changing that significantly would (will) require a large overhaul of the whole OS. If Apple is working on a tablet, then this isn't as big a deal and in that case, it could be sooner rather than later.
Camera - For two-megapixels, the current iPhone camera is great. However the competition is at four or five megapixels and has video. Look for Apple to get much closer to matching this in the next revision - the Broadcom chip would certainly allow for it. Better Camera
GPS is another feature many have been clamoring over. Recently, however, Apple/Google have come out with a "soft GPS" that uses known Wifi access points and cell towers to pinpoint the iPhone's location. It works amazingly well - and much quicker than my Nokia N95's GPS ever worked. However, GPS is more accurate and works in places where cell towers can't triangulate - so of course it would be nice to have. I see this coming when GPS is part of the SoC guts - which will probably be very soon. Also - WWDC is being touted as a "Landmak event"- that has GPS written all over it! GPS - YES
Stereo Bluetooth/Bluetooth Modem/Sharing? I hate to say it but Apple doesn't seem to care. I'd love to be proven wrong.
SD Card storage? Not Apple's game. Perhaps as the OS matures, external Bluetooth or WiFi storage will be available. It is a shame, though, because more cheap storage is always nice. However, Apple could forget about charging $100 more for a device with 8Gb more of Flash storage.
Software? The floodgates are about to open wide with the iPhone SDK unveiling this month. Besides the amazing selection of hacked iPhone applications already available for hacked iPhones, Apple is expected to announce Exchange and Lotus collaboration suite syncing. Additionally, Slingplayer and Skype clients are rumored to be in development (FRING!). Hopefully we'll see some sort of proper instant messaging client; the web and hacked versions all leave much to be desired.
Bonus? Those Broadcom chips and some others like it have FM broadcasting built in. Wouldn't it be cool if you could broadcast your music and videos to any radio in the room/car? It is doubtful that Apple would do something like this but something similarly cool always seems to pop up.
All of these are obviously best guesses, but hopefully it will help you decide when and if you should pick up that iPhone that you can't get out of your mind.