We were pretty sure we'd see upgraded iMacs introduced at Macworld, but according to the Chinese-language Economic Daily News (EDN) via Digitimes:

.. sources inside the component supply chain as saying that Apple plans to launch a new iMac all-in-one PC in the first quarter of next year, and the company's sole manufacturing partner, Quanta Computer, is gearing up to supply monthly shipments of around 800,000 units during the first quarter of 2009.  According to the paper, the new iMac will start shipping in January next year. Quanta declined to comment on client relationships, the paper added.  In addition to Apple, Quanta is also manufacturing all-in-one PCs for Sony and NEC.

"New" is much more interesting than upgraded video and processor.  However, much can be lost in translation -including our interpretation- and Steve Jobs isn't presenting, so we aren't going to be "bringing a change of underpants" to the keynote.