I have updated my short article on getting started with .NET Framework 3.0 and Windows Vista to reflect the recent releases of Beta 2, June CTPs and July CTPs. Tom has done a great job at explaining the choices you have in more detail.

For the moment I am sticking with using Windows Vista Beta 2 (and hence .NET Framework 3.0 Beta 2) on my productivity and developer machine. Although if you are just planning on using Workflow Foundation, then the July CTP of the Framework and Windows Vista look to be a good choice as this includes Release Candidate 4 of Workflow.