Scoble has a good post on the Whidbey Slip [Scobleizer: Microsoft Geek Blogger]
In it, he says "The idea that Whidbey is being slipped just because it needs to tie up with Yukon isn't wholly correct...." and I have to admit to blogging just that conclusion without having actually, you know, asked Microsoft for their side of the story. I suspect it to be the case, but I haven't done enough research to conclude it to be true.
Back in September, I thought that Yukon was being slipped because of Whidbey, and now I've blamed Yukon. The truth is, these products are so complex that it's possible that either is the source of showstoppers. But what I do know is that Microsoft values shipping software and that polishing minor features in production-quality components while another team struggles to hit their quality thresholds will not sit well with competitive Redmond spirits.