Purely by chance, I came across this lovely interview with Isobelle Carmody on the UK site Fantasy Book Review.
She confirms that the final Obernewtyn book will be out at the beginning of 2010 and that she will soon begin writing The Beforetime Chronicles. This series will be set in the lead up to 'the Great White' and will feature the beforetime woman Cassandra/Cassie/Kasanda who we already know from Obernewtyn. This is really exciting news!
I was also pleasantly surprised to find out that Isobelle began writing Obernewtyn when she was just 14; around the same age that I was when I first read it.