Here's an interesting idea...
As we already know, as of June 7th, the iPhone 3G unlimited data plan for $30 was removed from their system. According to AT&T store reps, it's no longer listed as a choice when trying to activate an iPhone 3G/3GS.
And, as we also know, if you're on the $20 unlimited EDGE or $30 unlimited 3G data plan on an iPhone 2G or iPhone 3G/3GS, respectively, you can be "grandfathered" to the defunct $30 unlimited 3G data plan when you get the iPhone 4.
Now, here's the kicker...
If you have an old iPhone 2G (not 3G or 3GS) laying around, you might want to try activating your number on it with iTunes. You'll see that you'll still be able to get the $20 unlimited EDGE plan during the activation process, and that may likely "grandfather" you to the $30 data plan if you choose to do so.
So, if you feel you missed out by not having an unlimited data plan on your number before June 7 changed everything, this may be the only way to "grandfather" yourself even now.