Poking around the forums, there have been several discussions on what cameras people are using, but not much on how to choose a camera.
What are the most important points to look for in a usb cam for multitouch? Resolution? Refresh rate?
I was planning to go with a cheap logitech for starters (good reputation, drivers available for linux) which has a resolution of 640x480 @30 fps. Is this good enough? Can I go with something even cheaper (320x240, or maybe only 15fps)

I understand that theoretically, higher resolution / higher frame rate should be better, but not having actually tried it yet, how much difference does it actually make? (also, the higher resolution, the more intense the blob finding becomes. At what point is it too much?)

well i went with the philips one because joobs and white got it and they had good results
i used a cheaper one that was about 25 euro’s, 4 years ago (sweex), so that one should be about 10 euro’s now or so and the IR thingie was easy to remove, the cam worked, but i bought the other one just because we had budget
But if i had to pay it all for myself i think i would stick to just a cheap solution, but thats just your call on how much you want to spend on the performance of your table

The difference between 15 and 30 fps is huge. I’m not sure about the resolution but I would definitely go for 640x480 at 30 fps if you can afford it.