"The Swedish gaming site Level7.nu claims that by finishing Ratchet & Clank: Quest For Booty, you’ll get to see a screen that says the next installment in the Ratchet & Clank series will be released in late 2009. For now, chalk this up as a rumour, but we wouldn’t be too surprised if this is true since Insomniac are churning out games like madmen."
http://www.thesixthaxis.com/2008/07/24/the-next-ratchet-clank-in-2009/
I think it's not out of question given the history of insomniac.resistance and ratchet every other year.