Using Visual Studio 2008 or 2010 for ArcGIS Desktop?

Go with 2010. I don't know any reason you'd pick 2008 over 2010 if you're developing with ArcGIS 10.

VS2010 adds support for .Net 4.0, so if you what to use new .Net 4.0 features in other projects choose VS2010. But for ArcGIS 10 projects you are forces to use .Net 3.5, so for those projects it does not matter right now. Place a vote here if you want ArcGIS to use .Net 4.0

VS 2010 so long you use the .net 3.5 framework. Both Standard and Pro will work (although the installer for the ArcObjects SDK check for VS 2008) Only VS 2010 Express is not supported only 2008 Express.