35 minutes of SD YouTube video
or 8-10 minutes of HD video
or 70 songs downloaded from iTunes
or a few thousand normal web pages and typical emails (no huge attachments)
or more than 4000 Facebook pictures
That´s per month usage.
I´ve read a article (lost the link), that stated that iPad Apps are self throttling their bandwidth over 3G, dropping in quality in the case of YouTube video etc, except Netflix which just one 30 minute TV show can use up one´s 250MB limit under the $15 AT&T plan.
I don´t see how AT&T will be able to handle the bandwidth of the $30 unlimited plan with people watching a Netflix movie or two a day plus web surfing etc. over 3G.
Even Comcast broadband is throttling high bandwidth users now, as the courts overturned the FCC ruling.