The iPad 3G's GPS is every bit a true GPS. In the case of the iPad 3G (and iPhone), assisted GPS does what a regular GPS can do, but more. Heck I can turn the 3G radio on my iPhone completely off and the GPS is every bit as accurate--outdoors.
The "assisted" part comes in handy in places you can't see the sky very well, such as indoors, and in helping to get a sat-lock faster. In those cases the iPad 3G can use the cell network and/or WiFi to help. Most of the reason for assisted GPS is because of emergency response: if you are in your house and call the police in an emergency, and there is no sat view, AND you hung up before saying where you were, it could mean life or death in this age where many are no longer using land lines. Yes the iPad is not actually a phone but it uses the same radio and baseband as the iPhone (3GS).