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 (http://www.google.com/products/catalog?q=3+way+motion+detector&rls=com.microsoft:en-us:IE-SearchBox&oe=UTF-8&um=1&ie=UTF-8&cid=8308451665827013991&sa=X&ei=TF5uTeecJcL7lweCoZSUAQ&ved=0CC8Q8wIwAg#)
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 (http://www.smarthome.com/2523W/Leviton-PR180-1LW-3-Way-Wall-Mount-Occupancy-Sensor-White/p.aspx)