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I checked to see if it was something to do with a link in a gmail email (BTW, gmail.com doesn't seem to send a referrer when you click on a link in an email), but it didn't show up.
Then thought it might have something to do with the smilies in the toolbar, but zilch, or at least doesn't show up in my logs. Ditto for adwords on a site. Maybe something on Froogle? Or a link from adwords in Gmail?
Might be worthwhile dropping Google a line asking for an explanation. Don't forget to share it with us!