Think about it, why would GoDaddy want you to buy a domain for longer than necessary? IMO it might have the slightest of impacts but it really pales in comparison with just one good link. Google has a white paper on this subject but that does not mean they use it. Screw GoDaddy, keep your money where it belongs, in your pocket.
I definitely agree that there's little impact, however... if a website is a serious business endeavour, chances are you'll keep it around for a while anyway... no reason not to register it for two years. if there's the most miniscule of benefits from doing so, all the better.
I somewhat believe the guys at seomoz are close to what I agree is a rough estimate of the google weighting factor. http://www.seomoz.org/blogdetail.php?ID=1467