Well that didn't take long! JK Rowling said on the Today show this morning she is going to write a Harry Potter Encyclopedia which will include inhanced back stories on many characters and tidbits and details not in the final book as well as things that ended up on the cutting room floor in the previous books
She hasn't started to write it yet, so it should be a while before it comes out.
If you're going to speculate as to what might be included take great care not to post spoilers under this thread (or face the wrath of Jason Evans)
I said before the release that I hoped she would do something like a handbook. Her website has offered some info, like backgrounds on less significant characters. I would love to know, knowing what she knows now, how long the first three books could have been?