On their tech specs page they have no mention of it: http://www.apple.com/ipad/specs/

I am guessing 512MB otherwise it would have been mentioned if it was 1GB. Kinda bummed by that, 1GB would have been nice.

They never say in the specs. We'll have to wait for someone to corner an Apple rep at the hands-on or iFixIt to tear one down.

I find it fascinating that the technology is so pushed to the background, putting the experience front and center, that they don't even mention how much memory it has. This is a very conscious decision.