This has happened to me with this site, and seems to be a common problem, but fear not, help is at hand.
According to Brendan Kowitz this is due to ASP.NET being unable to match the googlebot's browser signiture and thus defaulting it to the 'MOZILLA 1.0' setting.
He's posted a solution to the problem, and you know what, it works! The solution is a simple genericMozilla.browser file which you need to include in your site. It also solves similar problems with other search engines.
You can check what response google will receive from your site with this tool.
Through the tool I can see that my site now returns the correct HTTP/1.1 200 OK status message, whereas before it was returning a 500 error.
Hope this helps someone.