The entire situation surrounding the payment for 3G iPads has been weird. First, some people had account-checking charges right after they pre-ordered (I was one of them, with a $1 charge to my account). Then, the pre-authed for the entire price, right before the WiFi shipped. Not long after that, the charge dropped off. Fast forward a few weeks, and now some people have pending charges, while others are dropping off (mine dropped off yesterday).
Personally, I'm a little tired of Apple screwing with my available balance all the time. I've pre-ordered many Apple products in the past (iPhone 3GS, for example), and this has never happened. It's not that I don't have enough money in my account, it's that it makes my finances difficult to keep track of, especially when my balance is increasing and decreasing all the time.