If you are using the Visual Studio 2008 Beta 2 Virtual PC images, you will want to backup your data to another location (you should be doing that anyway). Microsoft is trying to get the word out about a major snafu - "that the current Visual Studio 2008 Beta 2 VPC images will expire on November 1, 2007, rather than March 15, 2008 as originally announced". Yikes, that will be a problem for folks that don't backup their data, as there is a good chance that after Nov. 1, you will not be able to get back into your VPC. So, if you are using the VPC images in question, backup your data. Definitely spread the word, and let others know about this problem.
I've install Visual Studio 2008 Beta 2 on my machine (not my production machine), so I'm not affected. But, I will tell you that .Net 3.5 Beta 2 does install some patches to the .Net 2.0 dlls. I know of at least one case where the patches fixed a bug in ASP.Net, and the developer didn't realize it until they tried to deploy their code to a machine without the patch. This is exactly why I love Virtual PCs, and continuous integration on "clean" build machines. If you are not using a continuous integration product like CrusieControl.Net, it is easy to get started, and the benefits are immediate.