Gonna be a while I think...GT200b was rumored for october release but pushed back until late december...Be lucky to see anything before June methinks.
Even so, another refresh? Maybe more SPs on the existing 55nm process. It's hard for nvidia especially to cram anymore into their monolithic design without going down to 40nm, so it may be hard to expect anything world-changing, challenging the 4870X2 with a single chip design would be extremely difficult on their current production process.