As unlikely as the rumour is that OS X 10.6 is going to debut at WWDC next week, both Ars Technica and TUAW have independently verified from their sources that the next version of the OS could be coming as soon as Macworld 2009. Ars also says that the code name is Snow Leopard, which on the surface latches on weirdly to the current Leopard name. But when you think about the rumour that 10.6 will only be performance upgrades, UI changes and switching to being Intel-only, that code-name kinda starts to make sense as a final label.
If we had to guess one way or the other, this no-feature upgrade doesn't seem to be something Apple would do. They gave away 10.1 to 10.0 owners because 10.0 was lousy and needed a bugfix badly, but 10.5 Leopard works just fine. They probably wouldn't waste their goodwill on their point upgrades being US$129-worthy by doing something like this. Rumour status: UNLIKELY. [Ars Technica]