There is an excellent recent thread named Canonical URL problems results in two sites in Googles eyes where Dayo_UK explains the current issues with having a domain name without 301 redirecting the non www to the www.
He uses Matt Cutts's blog as an example, saying;
mattcutts.com/blog/2005/10/ is not indexed under the non-www and if you query all the DCs you will see that it references the www on a "mattcutts.com/blog/2005/10/" search as the page.
So you would probably think that the Canonilization process is working correctly - and we would assume that Google thinks that mattcutts.com/blog/2005/10/ is the same page as www.mattcutts.com/blog/2005/10/
But if you do a PR check on the non-www and the www pages you will start to see that they are in fact split - the non-www has no PR while the www has PR accross the DCs.
Dayo_UK explains that Matt's site "survived the site split" effect of the canonilization process. But many sites are not. Matt most likely survived it due to the number of fresh inbound links to his site.
So if you have an ordinary site and the non www version is not 301 redirected to the www version (or visa versa) then you should probably look deeper into this thread and make that decision. You can easily check your header status at this or this tool.
Forum discussion at WebmasterWorld.