Via Robert Scoble's blog post entitled Microsoft postpones PDC we learn

Mary Jo Foley (she’s been covering Microsoft for a long time) has the news: Microsoft has postponed the PDC that it had planned for later this year.

The PDC stands for “Professional Developer’s Conference.” It happens only when Microsoft knows it’ll have a major new platform to announce. Usually a new version of Windows or a new Internet strategy.

So, this means a couple of things: no new Windows and no major new Internet strategy this year.
...
Now that Google, Amazon, Apple, are shipping platforms that are more and more interesting to Microsoft’s developer community Microsoft has to play a different game. One where they can’t keep showing off stuff that never ships. The stakes are going up in the Internet game and Microsoft doesn’t seem to have a good answer to what’s coming next.

Interesting analysis from Robert, I agree with him that Microsoft no longer has the luxury of demoing platforms it can't or won't ship given how competent a number of competitors have shown themselves on the platform front. The official Microsoft cancellation notice states

As the PDC is the definitive developer event focused on the future of the Microsoft platform, we try to align it to be in front of major platform milestones. By this fall, however, upcoming platform technologies including Windows Server 2008, SQL Server codenamed "Katmai", Visual Studio codenamed "Orcas" and Silverlight will already be in developers’ hands and approaching launch

This makes sense, all the interesting near term future stuff has already been announced at other recent events. In fact, when you think about it, it is kinda weird for Microsoft to have a conference for showing next generation Web platform stuff (i.e. MIX) and another for showing general next generation platform stuff (i.e. PDC). Especially since the Web is the only platform that matters these days.

My assumption is that Microsoft conference planners will figure this out and won't make the mistake of scheduling MIX and PDC a few months from each other next time.