So far this year, we've seen refreshes to the iPhone, iPod and MacBook lines. What's left? Well, the next round of speculation was started by a report that coming in the next few weeks will be a refresh of the iMac line. Based purely on the seasonal shopping trends, you'd expect this to happen well before Thanksgiving.
It hasn't been that long since Apple updated the iMac, though. It was late April, a Monday to be exact, and quietly done. Still, the 2008 roadmpa that AppleInsider has presented thus far has been pretty accurate. With that in mind, we should be seeing an iMac update in November.
If so, I'd expect to see a move to the Intel's Centrino 2, with faster Core 2 Duo CPUs. With Snow Leopard upcoming, they might even go to quad-core as an option. Perhaps they'd move more in the NVIDIA direction (ATI graphics cards occupy the lower end of the iMac line).
Since they just introduced the new MacBooks last Tuesday, you'd expect they wouldn't be looking for an update "event" for at least a couple of weeks (Apple's earnings results will be reported this Tuesday).
My guess: since you'd definitely want it before Thanksgiving, so perhaps the first or second week of November, at the latest.
We'll see. Nothing Apple-wise surprises me any longer.