When you go to apple.com/ichat you are taken to Apple's standard iChat site for Snow Leopard. When you go to apple.com/ichatmobile, you are taken to the same place. This URL seems to act as a place holder for a future product.
What kind of future product? Well, there are a number of possibilities. The first is that Apple is looking to release a iChat like web-app for iPhone's and iPod touch or an App Store compliant native app. Something along the lines of Meebo. Think of the way MobileME does Web email with the same look and feel as the Macintosh Mail.app. Perhaps Apple has a web based version of iChat planned.
The URL could also be evidence of Apple including a mobile version of their iChat software in iPhone O.S
