I heard the compatability was more around 98%. Even as such it is rumored that the software emulation may help to render the PS2 games in HD.
The US/Japan console are at 98% but with the removal of the hardware Sony has said the BC is "limited" for PS2 games(Anyones guess till they release. My guess 70% to low 80% is software only Europe does not get the PS2 hardware in their PS3's. People seem to be implying on this forum that they are going to drop under what the 360 BC is at which is close to 30%.