Not forgetting Gran Turismo PSP ... and as above LBP for PSP ...you may think LBP looks kinda meh but play it a few times and some user created content and you will be amazed how playable it really is and will have user created content like its big brother on the PS3 .....and like the amount of user created content out there for PS3 will keep it going practically forever! (last count they estimated over 700,000 PS3 user created content levels!)
oh and MOTOSTORM Arctic Edge, A whole new Resident Evil game next year and a few more Metal Gear games too, Monster Hunter Freedom Unite, SOCOM Fireteam Bravo 3 and a few others I can't remember!?
Add to this more DLC comming such as TV episodes and movies available from the PSP PSN store ...
If you have a PS3 and a PSP then Vidzone launches this Thursday (although Europe mainly to begin with) that you can remote play/stream over 10,000+ music videos to your PSP (see my post in the console section) a lot of people don't quite get remote play they think its limited to being around your PS3 ... its not lol ... with remote play so long as your PS3 is on standby ... you can REMOTE PLAY your PS3 anywhere there is wifi internet ..so basically ANYWHERE in the world you can access your pictures and music and soon Vidzone!
Not forgetting you can use your PSP for emulation it emulates pretty much any oldstyle cartridge based console snes,nes,megadrive,master system,game gear,gba,gb,gbc and even N64 (although not perfect) oh and a whole back catalogue of PSX games too!
A bucket load of homebrew stuff such as the popular Ultimate PSPTUBE
There's loads of good reasons to keep a PSP these are just a few