My guess is that going into the fall Apple will put on a big show for the OS update for the iPad, plus will roll out at the presentation several killer apps. If they really want to hammer away at the competition during the holidays then there will also be a price break.
So between, OS4, new flashy apps, and a potential price break, they'll have plenty of hype to sell a gazillion iPads for Christmas.
Thus, no need for a new version of the iPad this year.
The iPad really is the killer product for this upcoming holiday season. Unlike an iPhone which requires an expensive wireless contract to be useful, anyone can make use of the iPad, and so plenty of people will be eagerly buying them for spouses, grandmothers, etc. So in particular, to get a price break on the 3G model so that it is sub-$500 going into the holiday season is going to be key to Apple holiday hegemony.