Well according to a Nielsen survey (of 6-12 year olds apparently), 44% of kids want an iPad for Christmas. My kids are in that age range and are already using an iPad for fun and learning games daily.
It looks like Apple rounds out the top 3 with the iPod Touch and iPhone. There are some kids interested in some other tablet besides the iPad, but only a combined 25% of kids want "any other tablet". That is a pretty broad category since there are 20 or so other kinds of tablet besides the iPad.