Duchess of Aquitaine: A Novel of Eleanor by Margaret Ball (author of numerous fantasy novels), coming June 2006 from St. Martin's Press (link)
I reviewed this for Booklist's April historical fiction issue, but the review isn't online at Amazon yet. In truth, I'd been waiting at least five years for this book to be published. Amazon had it listed as "forthcoming" for about that long, but it never appeared. The author's weblog (via www.flameweaver.com, though the archives aren't working now) said that she'd turned the manuscript in eight years ago and neither she nor her agent knew what was going on. I enjoyed the novel quite a bit.
Abundance: A Novel of Marie Antoinette by Sena Jeter Naslund (author of Ahab's Wife and Four Spirits), coming October 2006 from William Morrow (link)
Lots of buzz about this fall title. I have a feeling it's going to be a fairly long and detailed book, given the author's other novels.
Queen of Shadows: A Novel of Isabella, Wife of King Edward II by Edith Felber, coming November 2006 from New American Library (link)
I know nothing about this one other than the title, but I believe the author is Edith Layton, who writes historical and Regency romance. This looks like a mainstream historical, though.