Planet13 - 10:01 pm on Jul 14, 2010 (gmt 0)
I find it hard to believe that just doing canonical link tags would decimate rankings like that... did you also do 301 redirects?
The reason I say that is that google has repeated often that canonical link tags are only "a suggestion" as to which url they should index.
By the way, how did you decide which was the correct canonical URL? Did you look at which version of the page naturally ranked highest for the keywords you were targeting?
Did you set the canonical URl to the URL that had the highest conversion rate?