Does anyone know of a site or reference source that will return the associated cell phone service provider given the area code and prefix of a number (ex: 804-519-xxxx would be Sprint PCS)?
Let’s assume that I posted this 2 days ago, before Wireless Number Portability began. 
Civil Defense, I don’t think the number portability issue necessarily has any effect on your question. Number portability doesn’t apply when you’re moving out of your local telephone service area.
Well, primarily I was going to use it to figure out which service the majority of my friends are using. That wouldn’t work well if they got their number from, say, Sprint and changed locally to Verizon. I’m guessing their prefix would still show up as a Sprint number.