As the recent discussion about Unofficial (read: our local anti-scholastic-campus-booze-fest) on this blog precipitated some discussion of how to perform the flip-off/middle-finger gesture, I feel I should say that, in my experience, I've seen this fuck-you gesture performed four ways:
1. with finger prominent and other fingers held back
2. with finger prominent and fingers rotated forward
3. with finger prominent, fingers rotated forward, and thumb extended
4. one of above ways but using middle finger to point at the recipient of said gesture.
Number 4 is particularly intense and not covered in Blank, Blank, and Moore's guide.
This Google.image.set gives a pretty good roundup of middle-finger-variation. This Flickr.set does as well. My favorite by far is this one: