I've paid off my 4S, and completed the authorized unlock. So I'm free to consider other options with for my GSM iPhone. That includes H2O Wireless (we use it for the kids voice/SMS) and Walmart's Straight Talk, an AT&T MVNO.
Straight Talk costs $45 a month for "unlimited" text, talk and data. According to the Terms of Service that does not include tethering for a "laptop or personal computer", but that does leave iPad and Nexus 7 in a gray zone. There's been at least one report, however, of Straight Talk terminating service for heavy data use. (Virgin Mobile doesn't allow tethering either.)
It's not impressive enough.
Mobile economics in the USA are challenging. I'd like to buy an unlocked iPhone at full price and then buy my voice and data separately, but currently the market is biased to the subsidized device model. It will be very interesting to see if any new options emerge with the iPhone 5 -- assuming it's LTE.
If an option does emerge, it better charge me for my data. I don't trust any vendor who offers me unlimited data -- those plans always come with strings attached.
Update: I might have been too hard on Straight Talk -- see this GigaOm review and Gordon's Tech: The US iPhone scene: Options and trends