And no, it's not because I think it'll eat into Mac's growth (average about 25% growth in units, 20% in revenue for the next couple of years). But by Sep 2012 (ttm) iPad would grab the #2 spot as a revenue driver within Apple, behind the iPhone. If we annualize the current quarter's estimate it already exceeds annual iPod revenue, which was the #3 driver up to now (this even after taking into account the December seasonality which benefits the iPod).
Back in early April, before few had any clue how big the iPad would be, I had this very ambitious forecast for it, along with all other Apple product lines. I was way off in underestimating its contribution (as well as iPhone's). Here's an update to that chart: