going to be implemented as part of the canonical API in 3.4.
Here is the rewrite that should do the trick for /login, /logon, /login/, logon/. You are welcome to play with this rewrite rule for maximum fun!
RewriteRule ^log(i|o)n\/?$ http://<strong>yourdomain.com</strong>/wp-login.php [NC,L]
Please change the name of yourdomain in the above code.
Happy Rewriting! :)