The Orange Box. That's like a whole PSN game in patches! We say "mysterious" because, well, it seems like nobody has any idea just what's included in that patch.
If you'll recall, EA took care of porting The Orange Box to the PlayStation 3 while Valve concentrated on the PC and Xbox 360 platforms; unfortunately, the PS3 release was not only delayed but it also suffered from some (perhaps exaggerated but nevertheless serious) framerate issues.
[Update: Official changelog from EA posted after the break, for your edification.]