Some time ago a Jon sent me a nice postcard. It contained a link to his blog where he wrote about aut-changing the display profile when an external monitor is connected. He wrote a lot about it here. I also had this problem frequently on my Thinkpad T60, so I will probably try this out.
Short summary: The IBM presentation director can run commands on a display setup change, which could also be used to change the profile by a dedicated xcalib commandline.