With the release of BuddyPress 1.0, BuddyDress.com has released an updated version of their FaceBuddy theme which strangely enough, looks just like Facebook. This theme is very CSS dependent meaning little to no images. One nice thing about this particular theme is that BuddyDress provides the PSD file to customize the background image with your own logo or text.
If you really want to have your own Facebook look alike, head on over and grab the theme. It’s free.
*Updated July 6th, 2010*
As has been pointed out by a commenter, this theme appears to no longer be available. Even when browsing the BuddyPress website and performing a search for the Facebook theme, the results point to the same link, BuddyPress Social which is part of their Premium theme selection.