Well, it's pretty stupid to leave it on for more than 24 hours without using some kind of external cooling. We've never had any problems with ours (knocks on wood), but then we're very careful about keeping it near the AC.
I doubt when Sony produced the slimline that they realized that their PD division would saddle them with 24-48 hour duty-cycles... but considering one of the Tekkens has an unlockable that only surfaces after 24 hours of straight playtime (that was a damn b*tch), I'm sure they realized that the PS2 would see on-times within that range.