Return to the Rain Wilds was confirmed at Pat's Fantasy Hotlist interview.
According to UK SF Book News interview this will be a single book.
Quote from UK SF Book News interview: "This is NOT a direct continuation of Liveships or anything like that. Readers will get some glimpses of characters they've seen before, but not a full fledged continuation of any character's story. I want to be very honest about that. It does take place chronologically after the events in Fool's Fate. And of course, things that happened in that book are going to affect things that happen in the Rain Wilds and Trehaug. That is how the world works."
"I think I've decided that it will all be in third person, told by four view point characters." Robin's Newsgroup
Drafts 6 and 7 of the books prologue can be read from Robin's Myspace blog.
The book will be published in the UK by HarperVoyager 6th of July 2009 with ISBN 000729025X. The US release date is scheduled for January 2010.