Uhh...that's now how the PS1 Classics page is structured on my PS3 apparently. Instead, the games are sorted by letter, and I checked "M-P" and couldn't find Metal Gear Solid there. Then I tried a general search for the game and still no luck.
Maybe if I wait a while it'll turn back up? Does my PS3 being white and/or 320 GB have anything to do with it?