The market is saturated, it's september, of course the iPad sales are slowing down.
The thing with the iPad is that, only rich people who are up to date with Apple released bought the iPad 2. But now that they all have their own, who's buying iPad 2s? Nobody.
In my "entourage", over the two first months of release, three people bought the iPad 2 because it's just been release. Then I never heard about someone buying one again (although I wish I could have one).
The iPad is not the iPhone nore it is a Macbook. It is a gadget that doesn't fill any need (like having a telephone, a music player or a computer), so only people with enough money can spend it in such a useless device.
If Apple cuts the price of the iPad 2 (which is not likely, because Apple as a strategy of never cutting prices) for thanksgiving and christmas, the sell will go up again, but then it will go down until the iPad 3 release