User's solution

One possible fix:

http://forums.microsoft.c...p;SiteID=1

Another:

http://forums.microsoft.c...p;SiteID=1

It turns out I was trying to load a Visual Studio 2005 Workflow project, but didn't have the proper engine installed.

I installed it from here:
http://www.microsoft.com/...laylang=en

Now it loads just fine!