Are you sure you're on the RTM version? I did a test WPF project and it never pulled up a .Net 4.0 only item. I used the System.Threading.Task lib on a 4.0 project, then downgraded it to 3.5. Soon as I downgraded, it failed to compile.
I know beta 2 had some speed issues but on RC and RTM, on any machine I put it on, it works rather nicely. Do you have any third party extentions installed?