Hello, John.
I haven't tried it on a back projected surface, I don't exactly know the physics here off the top of my head, maybe someone else will answer this part.
The line sensors on the 3pi are infrared based, so the infrared from the table will probably mean it won't work. You would have to try it out to see for sure. One time while I was doing line following on a course that was half in shade half in sunlight through a window, the 3pi would sometimes get messed up when it transitioned from the darkness to the light.
If you are concerned about the 3pi flying off of a coffee table sized space, you shouldn't be concerned. It can go slow enough to follow a line safely on a small table. Here is someone running it on his desk at work:
http://www.youtube.com/watch?v=ylyRi1Qlbl0. Here is an impressive example of what someone did on a small table (not really going too slowly):
http://www.youtube.com/watch?v=nAU_IQgiggM.
-Ryan