Microsoft drew a lot of flak when Xbox 360 debuted with a paltry list of compatible Xbox titles (which shrank before it grew), but at least there was an effort to be honest to the consumer. That's not to say Sony was being overtly dishonest by advertising 100% backwards compatibility; if we had to guess, a team wasn't budgeted to test all of PS2's 8,000+ titles. Sure, you could bitch that Sony did a disservice; but let's be real, anyone lucky enough to score a PS3 during launch probably owns PS2 -- problem solved.
*200 titles account for less than 2.5% of the PS2 software catalogue.
See also: A HUGE backwards compatibility checklist for the PS3
[Thanks Marc]