Hmmm, so you port your number to Google at the same time that you're asking your carrier to give you a different number?
This is just like any other port. So if you have only one line on your account that account will be closed.
Probably the best time to do this is if you're already going to port. Instead of porting to the new carrier, you port to Google. Then you never need to port again (in theory).
Sounds like they are rolling it out to everyone again. This could also be helpful if you move into a new service area and need a new carrier...you could port your old number to GV and that way all your friends can still call and text your old number while freeing you up to get a local number in your new area code.
What I'm waiting for is international use. I already use Google voice for texting, and I think it is better than any other text services for my iPhone Wannabe, also called an iPod Touch.