Josh over at Web 1 Marketing writes about using www versus leaving it off. What’s wrong with no-www:
Websites should use or at least always accept both formats. Regardless of which you choose, someone will always try the other.
At Freelock, we've noticed that www is a bit of a litmus test for how technical a person is--all of our more technical users omit www, while all of our less-technical users always include it. We've had to add www to several domain names of things like mail servers that are not general web sites, just because our users assume it's there.
Where we've really noticed this phenomenon is in deploying Joomla web sites. While you can see pages on Joomla from a variety of domains, links get rewritten to whatever domain you designate as the main one. This usually isn't a problem, except when people log into the first page they visit--they inevitably have to log in again when the domain changes, and they get confused. So as a rule of thumb, we set up sites using www unless our users are clearly web-savvy.