According to my agents (US, UK, and European), what’s really selling well right now are historical novels that center around a historical personage of the second order, that is, someone real who sat just outside the spotlight ... Publishers, sheep that they are, are looking for more of the same.
This is something we've all been discussing recently; you may want to take a look at the comments on both blogs (especially one on Kerr's blog: "Most often than not, historical fiction gets filed into some bodice ripping section in one’s mind"). Ahem.
To my mind, while it's true that this type of historical novel has been around for ages, it has really become popular in the last 5-7 years. Girl with a Pearl Earring was one of the first examples; The Other Boleyn Girl was another. What Galleycat doesn't mention about the other Mary Boleyn novel they discuss, Karen Harper's The Last Boleyn, is that it's a re-release of a 1983 historical romance; publishers wouldn't normally bring older romances back into print as mainstream historical novels if not for reader demand for novels of this type. That sounds like a publishing trend to me. The summer 2006 catalog for Berkley, which I've got at home, echoes Kerr's comment to some extent (as far as it affects women in the British and French royal courts) in their ad for Susan Holloway Scott's upcoming novel Duchess, which is about Sarah Churchill.
Also, since not everyone goes back to read older blog posts, I wanted to point out some comments that Julia Oliver sent me over email about her upcoming novel Devotion.