[ I really didn't want to write about this, but because many news sites (Slashdot, Digg) already talk about this, it can't bring too much trouble. ]
Google can fix this in many ways and will certainly fix it. Until then, it's a good idea to sign out of Gmail when you're not using it.
Update (after a day): Google fixed the security vulnerability.
Sara Thomas, Ajay Deshpande
3 hours ago