Now you can login to Facebook using your Google Account. Here's how to do it.
Chose "account settings" from the settings menu.
Click "Change" on the "Linked Accounts" section.
Google will ask you to verify this permission. Allow it.
After that, you don't need to login to Facebook anymore. Each time you login to your Google Account you will also automatically login to Facebook.
I think this is very helpfull. It saves a lot of my time.
You can also do this automatic login using your MySpace, Yahoo, and OpenID account.