I have heard anecdotally that it's not uncommon for large players to bulk-start instances and then kill all of the ones that don't have the latest hardware.
Update: I just looked at 4 random c1.xlarge instances we have running, and found 3 different types of underlying hardware:
1. Intel(R) Xeon(R) CPU E5410 @ 2.33GHz w/ 6144kb cache
2. Intel(R) Xeon(R) CPU E5506 @ 2.13GHz w/ 4096kb cache
3. Intel(R) Xeon(R) CPU E5-2650 0 @ 2.00GHz w/ 20480kb cache
Of these cores, the E5410 is from 2007, E5506 is from 2009 and the E5-2650 is a Sandy Bridge from last year.