Sitecore 6 was the victim of a bug in Microsoft .NET 3.5 SP1 – which meant that Sitecore 6 developers could not use Visual Studio 2008 SP1 or .NET Framwork 3.5 SP1.
You can read alle about it at Alexey Rusakov's blog
However, I just discovered that Sitecore just now released a patch addressing this very problem. I tested it, and it seems to work.
What you want to do, is download Update rev.080820 from sdn5.sitecore.net
Good work Sitecore :-)
