"The finger", so often raised in anger and occasionally in a long distance proposition, is, as they say, a whole nother story. Contrary to legends about the English inventing "the one finger salute" at the Battle of Agincourt in 1415, the gesture was already well known in ancient Rome, where it was known as the digitus infamis or digitus impudicus (the infamous or indecent finger). Literary references are quite clear (and blunt) about the meaning of the gesture. Caligula was said to be particularly fond of it, but we know what kind of guy he was. As with the thumb gesture, however, there is no extant drawing or description of exactly how "the finger" was deployed.
The thumb: http://www.ur.ku.edu/News/97N/SepNews/Sept29/thumbs.html
The finger: http://www.ur.ku.edu/News/97N/SepNews/Sept29/thumbs.html, which also includes a retelling of the Agincourt legend.
Bonus: an amusing and completely inaccurate theory of the origin of the most common four-letter English language vulgarity: http://historymedren.about.com/homework/historymedren/library/bletym.htm
Other interesting "straight dope": http://www.straightdope.com/index.html.