A million a month? Hmmm..... not sure. It depends if demand holds up once the launches are done. I suspect they'll have a bit of a slump over the summer, start picking up again just before the schools go back, pick up again in September / October when OS 4 comes out then run to Christmas. It's a hard one to judge because, much like the iPhone, I think it's a device that's going to rely on people getting hands-on with it to build demand.
One thought that did occur... just checked the last financial report for Apple and they sold 2.2 million computers last quarter. They're going to sell more than 2 million iPads in the three month period since it launched in the US (provided they can make enough). If, IF they can continue with those sorts of figures they've effectively doubled their computer market share (yes, I count the iPad as a computer, let's not have that argument again).