The BestBuy guy was wrong, here is Apple's
locations services on iPad page. I think that it makes it clear that the iPad does use GPS satellites to determine your position. "Assisted GPS" isn't a great term because different people use it in different ways. In the iPad's case it uses WiFi and cellular data to make getting a first fix on your location quicker, however your exact location is determined by real GPS.
The reason that the WiFi iPad doesn't have GPS is that the GPS chip is on the same daughterboard as the 3G chips. I don't know why Apple choose to do it that way but
my guess would be a combination of cutting costs for the WiFi version and the fact that the Maps App doesn't work without a data connection.