There was an internet rumor going around that the original Killzone PS2 game might get an HD makeover and possibly be bundled-in together with Killzone 3 as some sort of Special Edition or put up on PSN as added downloadable content. But so far, it's only been a rumor
"Welcome to the circus of values!"
Currently Playing: Crysis (360), Destiny demo (PS3), Roadblasters (MAME)
Yeah, they're probably going to try and release all the classics in HD first then just release the rest on PSN. But I don't doubt that they will release a system update that allows for PS2 discs to be played on the PS3. I mean once the PS2 and games are no longer in production.