The simple answer is "not much". Fumito Ueda just appeared in a panel at GDC and would only say the following about his next, hugely anticipated project:
"Well, probably it will be similar to the past... we are struggling. The essence of the game is rather close to ICO."
That was it. This nugget came from a conversation between Ueda and Grasshopper star SUDA51.
Get the rest of it from Shacknews.