The folks at Curmudgeongamer decided to test the backwards compatibility of both the 60 GB and 80 GB versions of the PS3. With graphs and everything!
Using Sony's Software Compatibility Search tool and his own library of 90 PS2 games, Curmudgeon's jvm found that the 60 GB was compatible with 98.9% of his games. All but one worked perfectly, or with "minor inconvenient issues" like corrupted audio or graphics. The 80 GB didn't fare quite as well: 12 of jvm's games were unplayable, giving the unit an overall score of 84%.