H2o
I just converted my iPhone 4 to h2o and it was very easy. $50 (tax included) for unlimited talk, text, and MMS +250 mb of Data.
1) Get an unactivated h2o precut microsim. $.99 shipped on eBay.
2) Call h2o before putting sim in phone.
3) Give h2o your Act Fast # off of the sim card that you get. (it will be on the credit card size sim card holder that should come with it)
4) Have them port your number from AT&T. They will need your password and account number for the account.
5) They will activate but not add money to the sim card. The process of porting the number took me approximately 18 hours.
6) Once the card is activated, go to the h2o site and "recharge" your simcard. (Note that your PIN is the last four digits of your ported telephone number.)
7) Once you add funds to your account, the iPhone will be able to make calls within a few minutes.
8) To get your data to work, go to unlockit.co.nz, create a new profile for h2o, and install.
9) To get MMS to work, you will need to jailbreak your phone and install an application called TetherMe, and then change a few of the network settings. Search YouTube and there will be a step-by-step procedure for this.
10) Enjoy getting more for less!
Also, although the h2o site says that you can activate on your own, I had issues with the site stating that my sim card was already activated. H2o confirmed that this was not the case and activated it. Plan on being on hold for 30 minutes, but it is well worth the wait to save a ton on your monthly bill!
-prashant