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OK, I figured out how to add a canonical link to the magento login page.  I put this in my local.xml file (/app/design/frontend/<designpackage>/<theme>/layout/local.xml) : </theme></designpackage>

<customer_account_login><reference name="head"><action method="addLinkRel"><rel>canonical</rel><href>http://www.site.com/customer/account/login/</href></action></reference></customer_account_login>

As for whether or not this will get rid of the crawl errors, I won't know until Roger tries again, will report back later.

that's an interesting idea, I'm going to post my solution for adding "canonical" to the login page after I test to see if the crawler will stop complaining as a result of that.

I am using the Magento shopping cart, and 99% of my duplicate content errors come from the login page.  The URL looks like:

http://www.site.com/customer/account/login/referer/aHR0cDovL3d3dy5tbW1zcGVjaW9zYS5jb20vcmV2aWV3L3Byb2R1Y3QvbGlzdC9pZC8xOTYvY2F0ZWdvcnkvNC8jcmV2aWV3LWZvcm0%2C/

Or, the same url but with the long string different from the one above.  This link is available at the top of every page in my site, but I have made sure to add "rel=nofollow" as an attribute to the link in every case (it is done easily by modifying the header links template).

Is there something else I should be doing?  Do I need to try to add canonical to the login page?  If so, does anyone know how to do it using XML?