From everything I've read & heard, the only real, viable, confirmed-working alternative to an AT&T iPhone voice/data plan, at least for U.S.-based users, is T-mobile. You will *undoubtedly* save $$ vs. AT&T, whether you decide to go with an Individual or Family plan, or with a contract vs. no-contract/committment plan.
As you may already know, Verizon and Sprint (and smaller competitors such as MetroPCS) are out of the question due to their use of the CDMA (older, non-GSM/non-SIM) technology platform.
Just keep in mind a couple things if you're considering using an Iphone 3G on T-mobile: Your data speeds - particularly when Web-browsing on Safari - may be *painfully* slow. For starters, you will be restricted to Edge (2G) speeds, since T-mob's 3G bandwidths are incompatible with those of the Iphone. Secondly, I can attest that a jailbroken Iphone 3G runs slower even on AT&T's network than a non-jailbroken one. My iPhone 3G was never lightning quick with browsing to begin with, and since jailbreaking, surfing using even WiFi has slowed considerably, and is downright annoying on 3G.
You may want to consider (as I am) selling your Iphone 3G on Craigslist, Ebay, or here on TIPB and then buying the 3GS to jailbreak/unlock & use on T-mobile. This still won't give you 3G speeds on T-mob, but given the 3GS' *much*-faster processor, I would think that at least your browser won't be at a snail's crawl either. ** Would LOVE to hear feedback myself from any current 3GS users on T-mobile regarding this **
As for pre-paid AT&T SIM cards, I'm fairly sure you can indeed use them for simple talking/texting. But data I believe is entirely another story. Even if they offer a pre-paid data plan, my understanding is that it's not meant to be used with the Iphone. And even if you could get away with it for a short time, they will eventually catch up to you as AT&T runs periodic IMEI "sweeps" that will identify you as an Iphone user.
At that point, all bets are off as they will (at the least) charge you by the KB for data usage (which adds up $$ REAL fast), try to force you into the Iphone data plan (you will get annoying, daily text messages from them regarding this), and possibly be able to shut down your iPhone data use entirely (with possible exception of WiFi?), since AT&T's "rules" regarding GoPhone users are more strict than those for users under contract.
** Would love to hear BlakeJackson's input (or any other AT&T rep on TIPB) on this last point **
Just my $0.02 on the subject...sorry in advance if my reply ran a bit long, LOL