I'm not suggesting to have an aggressive email campaign sent every other week: "Hey, we've noticed you STILL haven't signed up to PerlMonks. How come good buddy?" until people sign up or start sending angry replies. Simply one email when they post as a signed in guest. Again I think we could give posters a choice between signing in as a guest and still being able to post anonymously.
I believe there's something valuable about being able to say "Hello, I'm new here and just checking out the site. Here's my question:" Signing in as a guest could be that way. But maybe there are far more people like tilly that were very glad to have Anonymous Monk in it's current form before they decided to sign up and having a guest option would be more trouble than it's worth. I clearly don't know that answer. I just think it's worth exploring even if briefly. :)
"...the adversities born of well-placed thoughts should be considered mercies rather than misfortunes." — Don Quixote