If you send a mail to GMail from a different service (like Yahoo Mail) and include javascript in your body, GMail executes it.
A sample:
Subject: a
Body: asdfasdf<script>alert("asdF");</script>
I tried using document.location='http://www.cnn.com' and it works, GMail homepage automatically redirects to CNN. It's funny that you can't come back to GMail unless you disable Javascript, go to gmail.com and delete the mail.
Nice discussion at Digg.
Update: it's fixed.
