They haven't discontinued the iPad 2 (yet) and likely still have plenty of remaining inventory left to sell. Given all the hype surrounding the new iPad, its also likely they experienced a sales bump in iPad 2's as many people who were either holding onto the iPad 1 or hadn't purchased at all jumped in to take advantage of the discounting for the iPad 2.
Good 'slipstream' marketing strategy really.
For reference, there's a huge iPad 2 ad taking up most of the side of a large building at the Chicago Circle by the Loop. I pass it when moving from the Ike to the Kennedy.