Ive been on the basic package for 2 years and I can happily say they cast a blind eye to tethering, VOIP, instant messaging, downloading listed files and streaming video/audio - as for p2p I havent tried yet. Just dont go over your 1gb limit. I synced to google calendar regularly (used a G1).
I had a t-mob web n walk a while ago, the only actual restrictions i felt were voip, and nimbuzz being blocked. Everything else seemed to work fine, they can't detect tethering etc.