I was there today as well, had a great time meeting Jim again, and was pretty surprised about the short hair.  The question that Jim answered "I'm not gonna tell you" was whether or not Jim's daughter was part vampire.  He also mentioned a couple of other things:

1.  Title of the next book is Ghost Story
2.  I asked whether he'd be increasing the number of Dresden files books, given that he was a two novel / year writer; unfortunately, that won't be the case, and he'll be working on other projects to fill in the time between Dresden books.

Overall, it was a great time.  I was really surprised by the turnout, given that 3 years ago when he was in HB for White Night, the crowd was about half the size.