I am currently using a 3G iPhone (not jail broken) with a GoPhone Pay As You Go (just switched over today.) AND I was able to keep my number

.
The AT&T rep used the same SIM card and phone number and just switched it over to a GoPhone plan... which I had to put in a Nokia 2610 because "the 3G iPhone is not compatible."

I am going to have to pay for the ETF, but this a small price to pay.
So this is what I did (for those keeping score at home):
1) activated a 3G iphone on july 12th with 24 month contract
2) grabbed a nokia 2610 from my friend (he had one laying around)

3) went into AT&T store and asked that they cancel my 24 month contract and switch the number over to GoPhone Pay As You Go
4) purchased $100 pay as you go prepaid credit
5) took the iPhone SIM card out of the iPhone and into the Nokia 2610
6) left AT&T store
7) once the iPhone was deactivated and the Nokia 2610 was activated I switched the SIM cards back - great success!
8) called 611 from my iPhone and purchased unlimited MEdiaNet (you might want to do this from your Nokia or GoPhone so that you don't pay for data on your iPhone before you set it up)
9) viola! working 3G iPhone with unlimited data
I will also confirm that the balance screen does like to pop up randomly (about every 15 minutes, see attachment) when using 3G service. Also, visual voicemail does not work, in fact, there is no voicemail indicator at all now

. Hopefully there will be a jailbreak app to fix this at some point.
* TIP: It says that you can purchase multiple months of MEdiaNet at once... and it will just extend your service 30 days. So, in theory, you could set this up for as long as you want. Or you can just activate it every month.
* NOTE: I previously had my iPhone jailbroken, however, I upgraded to 2.0.2 and did not re-jailbreak my iPhone. Just a note, if anyone was wondering.