The way I interpreted it, the plan is basically like a pre-paid phone plan, where you get 250MB and/or 1 month, whichever ends first. So i took it to be that if i started with the 250 MB plan on May 1, and then on May 21, I ran out, I could either restart the 250 MB plan for $15 or jump to the unlimited plan for $30. Both of these options would reset the date to starting on May 21 and expiring on June 21.
I could be wrong about that, but thats the way that I interpreted it.
Last graphic:
http://www.apple.com/ipad/3g/