In the Rockford, IL area, how can I tell what cell phone company
someone has?  For instance, any number beginning with (815) 494-XXXX
is US Cellular.  (815) 621-XXXX is Sprint.  Is there a list of
exchanges for this area?

According to the recent changes with the FCC, a given extension won't
necessarily map to a cell service provider due to the ability to
"bring your number" to a new provider.

Anyone who might have had a valid list before this, the list is
probably as fluid as the whims of expiring contract holders.

if you go to 411.com's Reverse Phone lookup, you can get the answer
soemtimes.  http://www.411.com/10742/reverse_phone

If you have a specific exchange, you can look it up that way...

"The phone number "(267) 688-xxxx" is a Philadelphia, PA based phone
number and the registered carrier is Nextel Communications."

Thank you floundies-ga!  You answered my question.  :-)