T-Mobile and AT&T Pay-As-You-Go Broadband by: PrepaidWirelessGuy
That's interesting. From the Prepaid Wireless Broadband Plans page, you should be able to link over to their sites to see their respective offer details.
The T-Mobile plan isn't advertised as a prepaid broadband plan per se; it's a month-to-month plan if you pay full price for the modem. If you mention pay-as-you-go broadband when talking to the rep, they may be confused.
The response from AT&T, however, is a lot more shocking. Their product is most definitely being positioned as a pay-as-you-go broadband offer, and there was a lot of press about it, particularly noting that they completely copied/duplicated the pricing launched earlier by Verizon.
I'd be interested to hear more about what they said, and how that conversation unfolded!