Can anyone suggest a discount carrier that this phone will work with? My understanding is Tmobile uses a different frequency, and all of the discount carriers use T-Mobile or Sprint's networks (Boost, Virgin, etc.) Are there any discount carriers leasing the AT&T network?
I believe that StraightTalk (http://straighttalk.com) piggybacks off the AT&T network. However, they might not like you using that much data. American carriers generally restrict which devices you can activate on which plans in order to guestimate average data usage for that device. For example, Sprint charges an extra $10/mo for unlimited smartphone data over the unlimited data for other devices. So, StraightTalk might decide to disconnect your device if you use "too much" data and aren't using one of their devices. I'm not saying it's right to put restrictions on "unlimited data", just that they might decide not to serve you as a customer using a device they don't sell (and it might mean that you have a $650 iPhone that you can only use on AT&T).
It's also possible that StraightTalk doesn't rent 3G off AT&T. So, it might still just be 2G. For example, Sprint roams on Verizon, but doesn't roam on Verizon 3G, only 2G (except in former Alltel areas).
It should also be noted that over 24 months, the unlocked iPhone will cost you an additional $18.75/mo. So, you'd have to find a discount carrier that was a lot cheaper, not just a little bit cheaper. If you expect to upgrade your iPhone every year ($37.50) or 18 months ($25/mo), the unlocked's pricing starts looking worse. So, make sure to think about that too.
If you use T-Mobile, data will work, but the fastest you'll get is EDGE (at the moment, that's not actually as bad as it sounds; presumably because the network is less congested than AT&T's, the speed's not too much slower in 'real world' use). [Source: I'm using an unlocked iPhone I bought in the UK on T-Mobile in California right now]
I also use an iPhone on T-mobile. As jrmg said, you'll only get Edge since the 3G frequencies are different, but the speed is fine for most use. I also highly recommend T-mobile overall. Let's just hope something stops AT&T's buyout.
As someone who has used the iPhone on both, not having certain features enabled as a result were annoying as well. Such as visual voicemail (as one example). Sure you can live with it but its still an annoyance.
You get data, but at EDGE (2G) speeds. Which is just fine for almost everything you'd want your phone to do. I am using my iPhone 3G on the T-Mobile network and it is not an issue at all =)
