I would love to be able to use spaces in usernames, but this does not seem possible. Is there a writeup in the codex or somewhere that explains what characters are allowed and why it was restricted to that set or is it just alphas only and suck it up?

If I want users to use something other than a username with no spaces, is my only real option a plugin that allows users to login with their email addresses instead, but still requires an alpha only username at account creation?

Oh, and I am on a WPMU/network site.