This may be buried somewhere in the documentation, but I wanted to mention it. .NET 3.5 Service Pack Beta 1 requires Vista SP1 if you are installing it on Vista. I know this because I was installing it into my VPC with Vista and the install has errored out. Then I saw a post about this somewhere, think it was an email from my MVP lead. So, if you are installing the Beta 1 SP1 on Vista, check to make sure that you are using Vista with SP1. IIRC, the post I read said that Vista SP1 would NOT be required on the final .NET 3.5 SP1. This is why they have betas.
Wally