All the advertisements in this thread are just poorly ran scam operations. Especially the "ReversePhoneDetectives" above my post is merely an affiliate with Intelius whom gains a profit for every customer they turn over.
If you want information on a cell phone, and you're reading this, stick with finding the Caller ID.
i.e.:
1. Use a free demo of a service such as www.telespoof.com
2. Call your own cell phone, and for the "friend", enter a phone you're near by with caller ID enabled (Such as your home phone).
3. Place the call
4. Watch the caller ID, and the spoofed number should show the original callers ID.
Note: http://www.num2name.com/ is a service with a limited caller ID database that can be used as well. I highly recommend not, however, as the most likely return result is "Unknown".
If the caller ID shows up as "Cell Phone", "Out of Area", etc, then you can also check www.whocalled.us (Free directory of numbers, in which users submit complains on annoying calls from various numbers - rather large database).
As a last resort, www.cellpages.com is an opt-in cell phone directory worth taking a peak into.
That's pretty much the legal ways of finding a cell phone owner's information. Good luck.
