I fall into the grandfathered category, but I am still getting a Verizon iPad this time around.
1) AT&T may drop the unlimited at any time
2) I seldom go over 1 GB and only went over 2GB once
3) Verizon's data plans are much more cost effective given my usage. I can pay $20 a month most months, and my rare overages are not that expensive.
4) Verizon has better connections most places I go. AT&T's speed advantage is of little help when I can't connect.
5) I have an AT&T iPhone 4, so having connectivity from each carrier provides some assurance that in moat places at least one devicemwill work
6) the CDMA disadvantage of no simultaneous voice and data doesn't apply to iPad.
7) I'm keeping my old iPad for my family, so the plan will live on if I need to use it.