I am a big believer in tiered storage. I recently spoke to a storage architect who claimed that one of their best decisions with an amazing economic impact was having tiered storage. In their case, they are using tier one and tier two storage systems. However, there is also a great deal of value in having tiered storage within an array as well.
As much as I am a believer in tiered storage, I am an even bigger believer in intelligent tiered storage. I define "intelligent" as being able to set policies for movement based on some metric -- such as frequency of use. And then being able to move that data transparently and online. Movement should be in both directions -- demotion to a lower tier if it isn't being used and promotion to a higher tier when it is.
This is one of the core value propositions of Compellent Storage Center with a feature they call Data Progression -- it is a highly virtualized storage system that supports differet disk types including FC and SATA. Additionally they support different RAID levels creating even further distinction between tiers.
Compellent has the unique capability to move blocks to and from different drive types -- FC and SATA -- and RAID groups (e.g. RAID 10 and RAID 5). This is all done transparently -- the blocks are all still part of the same virtual volume even as they span different RAID groups and drive types. Compellent can do this because they keep metadata about every block -- which allows them to keep track of each block and its associations. And they built a policy engine around all of this to make it extremely easy to use.
Compellent actually has an easy to use, transparent and valuable ILM capability that requires no additional software. And it supports all types of data - database, email, files, etc. This is great technology that becomes even more valuable in a tough economy where every dollar literally counts.