Replying to djcp:
I suppose you could do something stupid like enter "*.com" and allow anyone with a .com address to register in your multisite install, but c'mon. You can't fix stupid.
:-) I like your thinking. (That said, I see a legitimate use case in *.gov or *.edu for certain themed networks, for example.)
We use tabs, not spaces, and if you can re-upload it with a diff or patch extension, it'll be presented better in Trac.