Hi Mike,
I find my 70-200mm f4 (non-IS) on my 5D2 an absolute cracker for portraits! I know you mentioned you want f2.8, but I find within that telephoto zoom range and on full frame, you get really nice shallow DOF. Shooting at f2.8 is obviously more preferred but f4 on FF is no slouch. f2.8 leaves little room for error so you must nail the focus. Plus you can get it brand new for ~$650
If you can stretch the budget a little more ($860) you can get the king of primes 135mm f2 from DD Photographics. It also doubles up as a great astroimaging lens
Otherwise the Canon 85mm f1.8 is supposed to be a great performer as well and for the price of under $500 you can't go wrong.
Hope this helps.