Why would Apple leave its own party to show up at one that is arguably Microsoft's? Here are a couple of guesses.
With the iPhone, iPod, AppleTV, Macs and OS X, Apple seems ready to reinforce its position as a consumer electronics company, and not "...that company that makes iMacs."
Also, consider the number of products that Microsoft has featured that never had a life off the stage. Here's Apple's chance to directly contrast such potential offerings with real products that are ready to ship.
Of course, this is all conjecture and one blogger's best guesses. We heard rumblings of this rumor last month and the consensus was a clear "No way." Honestly, I still believe that Apple will focus on smaller, Cupertino-based press events, and I'd be surprised if they showed up at CES. But what do I know? I still use a Newton.