Just for grins I did use the dx web updater... it didn't do anything (since I had already updated with the full download).
I have VS2010 Beta 2 installed... which gives me .NET 4.0 (which of course includes .NET 3.51 SP1 )
The .Net Framework 4.0 is not based on the older frameworks. It is possible that VS2010 comes with with a current version of 3.5 because you can target older frameworks with it too, but i won't guarantee that.