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Migrated to http://ubuntuincident.wordpress.com/2010/10/15/dropbox-installation/.
Links
Troubleshooting
Error message:
W: GPG error: http://linux.dropbox.com karmic Release: NO_PUBKEY FC918B335044912E
Solution:
sudo apt-key adv --keyserver pgp.mit.edu --recv-keys 5044912E
More info at http://forums.dropbox.com/topic.php?id=18389 .