It's not that I don't like Ubuntu. I do. I just stayed focused on the full question, which was: "If an SMB wants to upgrade from XP, what Linux variants would you recommend? Consider this would be for an SMB with limited in-house tech expertise."
Note that last phrase: "limited in-house tech expertise." There goes Ubuntu. Yes, Canonical, the company behind Ubuntu, does offer professional technical support, but Canonical is still new at the support business and its offerings are rather generic.
My answer is that those requirements pretty much narrow it down to Novell and SLED (SUSE Linux Enterprise Desktop). Novell, and its resellers, knows support and SMB.
In particular, if I don't really know Linux that well and I'm running an SMB, I want a company that can offer me the full support package. That's more than just 24x7 phone support. Both Canonical and Novell offer that. Novell also offers other support options such as certification, training, consulting, and even retaining the services of an engineer.
I also can't help but notice that Canonical appears to be charging $900 for 24x7 support per desktop per year while Novell charges $220 for the same deal. Yes, of course, if you know what you're doing, you don't have to pay any of that. This is Linux after all. But, that wasn't the point. This is for a business that doesn't have a Linux maven on call. Once they do know how to handle Linux, then Linux's cost savings go from good to great.
Of course, SLED 10 SP 2, the latest version, isn't cutting edge Linux. The next version, SLED 11, is due out next year. But, again, what does an SMB want with cutting-edge any thing? I want the newest and neatest, your usual SMB wants a rock-solid operating system without any surprises.
Last, but never least for most businesses, SLED is the most friendly of the desktop Linuxes when it comes to dealing with Microsoft Windows servers. Many people don't like that Novell and Microsoft have gotten all buddy-buddy over the last few years, but if you want Linux and Windows to co-operate in the work place, SUSE is your best choice.
Another option for an SMB is to swing a support deal with an OEM. For example, you can swing a desktop deal with Novell and HP.
There are other options as well. More and more regional and local VARs (value-added resellers) and system integrators are offering desktop Linux support. Personally, I've always preferred working with local businesses.
But, if that's not an option, for an SMB, I'd go with Novell and SLED. It may not be an exciting choice, but it's a very practical one and for a small business, it's all about being practical.