Visual Studio 2008 sp1 introduced background compilation so it can underline errors while editing (like Word underlines misspellings). It's been in Resharper for ages. You can have both Resharper and VS error underling- they don't conflict.

In Visual Studio 2010 it's off by default (and Resharper 5 isn't released yet), so you need to turn it on. It's Tools-Options-Text Editor-C#-Advanced. Check underline errors in editor and show live semantic errors.

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.