Hm. General Antilles said "$50 [per month] at least" (in the IRC log)

Nope, $20/mo. if they remain unlocked non-carrier devices. You can get a pre-paid SIM for AT&T Go Phones (sold cheap on eBay), and get the $20/30days unlimited data add on. You just have to remember to re-load the SIM every month, and activate the unlimited data option every month.

At least, as of a few months ago, people were still doing that (putting it into unlocked Option GT 3G ExpressCards). No telling how long before AT&T closes that loop hole. But, you can also get unlimited "data only" plans for your phone (basically to use it just as an internet browsing device), through AT&T, that are as little as $35/mo.