Hmmm. My suspicion is that Apple doesn't want the iPad to have 512 MB yet. Remember the argument we had with some people here about the amount of memory? We were saying that 256 wasn't enough, and they were saying that as Apple has 256, it obviously was enough. Well, I think the fact that the iPhone 4 has 512 shows that 256 ISN'T enough. It's why, apparently, the new video editing app won't work (according to Apple) on the iPad, supposedly even with iOS 4; not enough RAM. It's not the cost of the RAM.
I think that it was deliberate. Despite what some have said, it's obvious the A4 chip package can have more than 256. The RAM is just another module in the package.
While editing on a phone is great, it won't compete with the Macbook for sales. Editing on the iPad would. I believe that until Apple decides exactly where the iPad sits within Apple's sales line-up, they don't want it more directly competing with the more expensive model.
While I hope I'm wrong, I don't expect to see 512 until next year.