This was just plain weird, or me being stupid. 20 minutes of my life, gone. Just like that. Snap!
I had setup a new Community Server 2.1 site on my office network with the SDK. Compiled the solution, loaded up the home page. Sweet! Then I added a Class Library Project for some custom work I wanted to do, referenced the new project, coded a bit, compiled, loaded the site.
Could not load the file or assembly "CommunityServer.ASPNet20MemberRole."
Wh'?
There's an obvious solution to this, of course, but the fact that it was triggered when I referenced my new project bamboozed me. I added the \lib\2.0\aspnet20memberrole.dll as a reference and still got the error. Long (boring) story short, I dragged the darn cs.aspnet20memberrole.dll into the \bin directory and that did the trick.