Yep a video capture card would let you use the video out on the PS3 into the capture card so that it can be streamed to the computer or saved for replay - http://www.associatedcontent.com/article/1766766/how_to_record_video_from_ps3xbox360ps2wii.html
It's a brilliant idea, but way too expensive, and too much work just to upload screenshots onto your PC, when you can have acceptable pics, just by using a camera.
I gave my Red, Yellow, and White AV cables to my friend when his broke, since I use HDMi cables anyway. So Im already down. Going out and buying all those just for screenshots is a bit too much. I admire the dedication of whoever did it though.
Edit: I think all games should have a system like Bungie's
Theater. Where the pictures you take, or the videos you make are saved onto the Bungie server, and can be accessed via Bungie's Xbox server, or PC, if you just go to Bungie.com, enter your details and download the picture you took in-game. It works really well.