Word is that while Sony has claimed the PS3 is powerful enough to handle full PS2 emulation, the actual software isn't ready yet, so they'll use PS2 chips as a stopgap while they finish up.
According to Ben, only Atari 2600 and Colecovision games are supported for the time being, although emulation of other systems will apparently be possible through software upgrades (it'll also function as a basic PMP).
While we saw the OLPC XO-1's OS emulation earlier this week, not everyone (including us) took the time to set up the necessary software needed in order to run it.