Well arent you glad all iPads are unlocked?
AT&T cant pull this crap without the carrier monopoly it has on the iPhone, people will just cancel their plans (remember they're month-to-month, no contract) and move to someone else.
Unlocked yes. But that's pretty much meaningless in the US. The other GSM carrier, T-Mobile doesn't yet offer a micro SIM, and more importantly their 3G network runs on a different frequency that's not compatible with the iDevices.
AT&T has a defacto monopoly, and in their usual fashion, is using it to hose their customers.