A while back, Google did have lots of trouble with canonical variations. Even today in certain "edge situations" they still can - so attention to canonical issues is definitely a best practice.
But when it comes to common platforms such as WordPress and vBulletin, Google seems to have already adapted. As earlier posts said, some sites may see a 10% boost over time, but others can't even be sure that anything changed.
If both versions of URLs have the same toolbar PageRank, then Google has already got it figured out, in my expwerience. In that case most of the advantage will come from more efficient crawling. If you see toolbar differences, then you may get more of a boost over time by taking care of any canonical issue.
By the way, with-www or no-www is far from the only canonical challenge. Check out Canonical URL Issues - including some new ones [webmasterworld.com] for something like 40 that have been noticed in the wild.