I find it mildly interesting. This plan would be good on iPhone. I think AT&T decided to go along with Apple so as to not lose the exclusivity on iPhone. They might lose money on it but not much because very few people can afford another data plan (in addition to broadband and iPhone)
Coverage is so bad where I spend my time, here in the very rural highlands of the central Appalachian region, I haven't yet had any desire to throw away money on a contract for a data plan. Even voice cell phone coverage is often spotty to non-existent, and 3G coverage exists primarily along the region's Interstate highways in areas I frequent only occasionally. However, during 2 or 3 months of the year I do work (and play) in more "civilized" areas with coverage, during which time buying 30 days worth of access @ $30 would be much cheaper than any other option currently available here in "Podunk".
This might make me seriously consider the iPad, if I can find suitable ways around some of the limitations of the iPad and/or its walled garden, etc.