It will have been about 8 months at WWDC since they released 10.5, so it would make sense to begin talking about 10.6. Given that they are almost certainly going to be discussing development of the iPhone OS at WWDC, they are about halfway through their purported development cycle of new OS 10.x's, and they announced the WWDC with a picture of two bridges coming together, it would make a lot of sense for 10.6 to be a big topic. It probably won't be even a half-disclosure of what they're planning to do with 10.6, but the tie-ins with the iPhone will be a big topic.
Bring on .6!