The Times newspaper in the UK reported this morning that in JK Rowling & Stephen Fry's interview on BBC Radio 4, to be broadcast at 9:00am GMT on December 10, Jo said that she's planning on writing another children's book after she finishes Harry Potter 7:
"There is another book that's sort of smouldering in a cupboard that I quite like which is for slightly younger children. There are other things I'd like to write. But I'll need to find a good pseudonym."
But just to reiterate - number seven is indeed the last in the Harry Potter series.
Got from veritaserum.com