It seems that Google has finally gone on the record that longer domain name registration has no impact on your search engine rankings, at least not at Google. A Google Webmaster Help thread has a post from Googler, JohnMu, who said outright that it doesn't make sense for Google to use this as a ranking metric.
Let me quote John:
A bunch of TLDs do not publish expiration dates -- how could we compare domains with expiration dates to domains without that information? It seems that would be pretty hard, and likely not worth the trouble. Even when we do have that data, what would it tell us when comparing sites that are otherwise equivalent? A year (the minimum duration, as far as I know) is pretty long in internet-time :-).
I think that is pretty convincing that Google doesn't use the domain expire date as a metric.
Forum discussion at Google Webmaster Help.