Google Makes Voice Number Porting Available to All
While policies can vary, generally it's like pulling teeth getting smaller prepaid carriers to release your number quickly, and with some (Tracfone) it's almost not even worth it.
This way, you use your Google Voice number, so if you decide to switch carriers, you don't have to worry about porting, you just change stuff from google's website.
And if you have a smartphone, this basically means you have unlimited SMS (no MMS yet), which can save you up to $20 a month.
Is it possible - AT&T
And for what its worth, my contact extension was done in December at Best Buy, mainly because the local AT&T agent was out of stock.