I'm grandfathered in the unlimited plan. I usually use <2GB each month, epecially when not traveling, so could conceivably save the $5/mo by switching to the 2GB plan. However, one of the reasons I don't use more data is bc 3G frankly sucks for video streaming. Once an LTE ipad model ships I am pretty sure I'll do more streaming and eat more data. I just have to hope that I can bring the unlimited plan with me as I did from the iPad 1 to 2.
... It will inevitably be more, higher bandwidth available means Netflix and other streaming will detect higher bandwidth and send higher quality video, etc. thru. Hope we still have unlimited data at $30/mo with the iPad 3.