phranque - 9:05 pm on Mar 17, 2010 (gmt 0)
in the recent Matt Cutts Interview by Eric Enge [stonetemple.com] the rel=canonical tag was discussed extensively.
this probably best sums it up:
Typically, the crawl team wants to consider these things hints, and the vast majority of the time we'll take it on advisement and act on that. If you call it a directive, then you sort of feel an obligation to abide by that, but the crawling and indexing team wants to reserve the ultimate right to determine if the site owner is accidentally shooting themselves in the foot and not listen to the rel=canonical tag. The vast majority of the time, people should see the effects of the rel=canonical tag.