Sadly, Apple seems to be reducing this to ~ 3 years (standard for consumer systems, though Microsoft goes past this for OS support, as without that, they'd loose money).
On the MP's for example, the '06 - '07 systems are EFI32. They lost support for some of the more recent graphics cards in 2009 (nVidia cards are EFI64 based, and will not work). ATI's products use EBC, which is why they've continued to be usable.
10.6 is an interim product, as it contains support for both K32 and K64, and Apple has stated they're not going to do this for long (they need to get the libraries converted to 64 bit). Once done, K32 will vanish.
What you have to remember, is Support = money, and Apple is cheap. Nor do they make anything on supporting old hardware, so it's in their best interest to reduce the support period in order to "encourage" Mac users to upgrade their machines more often (where they do make money).
If they've finished the libraries, then K32 support will be gone (ridding OS X of it, will be cheaper and easier for Apple). If not 10.7, 10.8 for sure. As it's already mentioned, the 2010 systems default to K64, which is a significant indicator as to the direction they're going IMO (and have stated they're going this route - it was on the front page of MR at the time - , which seems to have been forgotten).
