Actually the amount of RAM on board is dictated by battery life. Unless there was a breakthru in battery life for maintaining a volitale RAM's state, I would suspect the iPad (3rd Gen) will still have 512MB.
Also the doubling of the RAM will not provide any benifit to the overall operation of the iPad. Since iOS automatically rolls out applications not in use, as an app needs more memory it simply has to request it and it will get it within a few millisecond. There are no apps, that I am aware of, that come even close to needing 256 Megs of RAM. Even the games that are 1 GB in size only load the module in use by the user into memory.
Another line of thought is the push to have "universal apps". The iPhone 4S is already struggling with battery life and it doesn't even have LTE yet. Going to 1GB of RAM would further drain the battery with no benifit. So a developer is going to have to weigh how they code going forward with 3 generations of iPhones to support and 3 generation of iPads.