From http://www.commandlinefu.com

Use a decoy while scanning ports to avoid getting caught by the sys admins.

$ sudo nmap -sS 192.168.0.10 -D 192.168.0.2

Scan for open ports on the target device/computer (192.168.0.10) while setting up a decoy address (192.168.0.2). This will show the decoy ip address instead of your ip in targets security logs. Decoy address needs to be alive. Check the targets security log at /var/log/secure to make sure it worked.

For my part, since I run I.T., I’m not concerned about being seen by the sys admins, but if I was in an internet cafe, or coffee shop, or using a hijacked wireless network this could come in handy – just to run a decoy to gain some time.