Apple is notorious for launching products on the same day of unveilings. Like the poster above said, it would kill current sales.
Additionally, Apple could choose any day they wanted to make this announcement. They chose March 2nd. What would be the point of announcing March 2nd, only to say, "oh, by the way this device will be available in 2 weeks?"
On the other side, Apple can do pretty much whatever they want, so we will have to wait and see. My money is on WiFi versions going on sale immediately (3G not sure). Another litmus test will be leaks appearing a few days prior. Seems Apple has struggled to keep things under wraps lately as the announcement date draws closer (think iPhone4 & MBP). Keep your eyes peeled.
I just bought 2 iPads (32GB models) on February 18th. I'm taking these bad boys back on March 2nd after I find out what Apple is doing.