An interesting post from Elliot compared the number of searches in a month on Google versus the number of visitors received on type in traffic domains. He used the Google Keyword Tool and looked at the traffic of domains to compare the two. The results showed that longer domains received less type in traffic as a ratio to Google searches relative to short domains.
http://searchengineland.com/google-search-volume-compared-to-type-in-domain-traffic-32136