I have a series of lights in the basement that are connected to two 3 way light switches. I would like to replace one switch with a motion detector switch. Is it possible to replace only one switch with a motion detector or do I need to replace both?

Something like this:
Cooper Wiring K6107W 3 Way White Motion Sensor 150 Degrees
CFL's may be a problem, may need a relay if so.

I see no reason why you can't install one 3-way switch that's a motion detector and leave the other 3-way as a traditional switch. The motion detector could be something like this:
Leviton PR180-1LW 3-Way Wall Mount Occupancy Sensor, White - Smarthome