It really comes down to a few factors:
1- Your individual ability to tolerate exercise induced stress and still recover
2- Your attention to diet and recovery (sleep!)
3- How truly taxing each session is going to be (usually higher frequency necessitates lower volume each session).
I'm currently hitting chest and legs each twice every 6 days. This is the split I followed during Fall 2009 for several months before starting my 2010 contest prep, and while I still think my legs can improve a lot, the difference afterward was certainly noticeable.
Thibs has written 'specialization' programs where you pick one body part and hit it 3x each week, with other bodyparts put on a sort of maintenance routine (for recovery's sake). Again, it's going to be different on a person by person basis, but of course you never know if something will work until you intelligently try it.