Vimeo has a great login screen that reinforces the community approach to video sharing that they're all about.
Depending on where you are in the site, you're either given a lightbox version (pictured below) or a very open, very vibrant full screen view. In both cases, the design really draws the user to the log in fields nicely, without the beauty and vibrancy of the imagery taking a backseat.
As a nice added touch, the full screen version of the login page incorporates a whimsically floating cloud. As an even nicer touch it doesn't use Flash to do it.