I'm just going to throw out all the ideas I can think of right now, so it will probably jump around a lot:
The PIR sensor I found was at http://www.futurlec.com/PIR_Sensors.shtml (one of the $6.90 ones). From the looks of it, it will be able to provide a 1 (digital) when it detects someone moving, and with the small size they could easily be mounted discreetly on the couch. They have 140 degree range, so using 2, we can cover every location near the couch.
Fuhs suggested that we could use 3 to detect where someone is located near the couch. We could do something where it would detect someone approaching and get "happy", and if it detected them leaving without sitting on the couch it would get "sad".
We could also try out using fiber optic cable to create a large amount of smaller point lights. These have the advantage of easily being able to create lights of varying intensity (for if we wanted to do the star idea), but might be impossible to use with the couch.
At this point, we can't really figure anything out regarding specific lighting ideas, since we still don't know how the couch diffuses light, or how we can mount lights inside it.
For driving any LEDs we might use, we can pretty easily run a near-infinite number of them using some LED drivers (and there is already arduino code for this, which could easily be expanded to however many LEDs we want). It's pretty easy to get a driver that can be dimmed, but it will dim each LED hooked up to it (so there would be groups of 8 or 16 that have to be dimmed the same). This might be fine if we just want to create a certain RBG color over the couch or a certain area (run all red LEDs together, and dim them together, etc), but might not be sufficient if we want every LED to be completely independant.