goodroi - 12:55 am on May 12, 2011 (gmt 0)
I came across a website with canonical tags setup on all of their pages and they were pointing to a spam site. I suspect someone hacked in and changed the canonical tags to siphon link juice. Now that cross cross-domain canonical tags are supported I would not be surprised if this becomes more common.
The canonical tag is a small line of code that is easy to overlook despite its large implications.