To a certain degree, I don’t think it makes any sense for the PS3 to have a UMD slot…at least not for watching movies. Who in their right mind wants to watch a 480x272 UMD movie on their high definition TV set. Even the person with a standard tube won’t want it since they can get the higher resolution DVD (720x480) for less.
Perhaps the UMD slot would allow you to play your PSP games on your TV like the GB player for the GameCube. However, this will depend on the titles being released. If they’re mostly releasing ports of games then it’s not worth it. If they have really unique games for the PSP then it will make more sense.
It would be nice if the PSP does connect to the PS3 like the GBA/SP connects to the Gamecube.