The release candidate of Visual Studio 2010/ .Net 4 is out at http://msdn.microsoft.com/en-us/vstudio/dd582936.aspx
You have to uninstall VS2010 and TFS 2010 before installing, which has two reboots.
The main changes are apparently performance improvements. I spent 20 minutes playing with my (small) projects and didn't notice much difference. The splash screen (now a darker blue I think) sits there for ages before VS2010 appears, but that's no worse than any Visual Studio version before. The real test will be building large multi-project solutions which I haven't tried yet.