bluntforce - 7:10 am on Mar 6, 2013 (gmt 0)
I signed up fairly recently with a firstname and a single letter as lastname. Email address provided by me was email@example.com
Using the previously posted example given, it would be similar to firstname.lastname@example.org
I didn't have any issues with signup and it seems functional as I've received Circle requests or whatever it's called. I'm sure there was some reason I took the plunge on signing up, I just don't recall what that reason was.