Even if slightly expensive, you can use NetworkLocation to autodetect your location, and after configuring it for your home network, add
Open an Application / Run Script and configure it so that it runs this StartInternetSharing script. Repeat, only this time check the box saying
Launch when leaving location and add the StopInternetSharing script instead.
Scripts weren't written by me, but I've updated them and hosted them on my Dropbox because I have a checkbox for Xgrid, and so the original version wasn't working. I've tested them and they both work. Here is the MacRumors post where these scripts were originally posted.
P.S. I don't know if I have Xgrid because of something I installed, or everyone with 10.6 does. Could you read/follow the instructions in either script's headers (open in AppleScript Editor) and let me know if you do? I'd like to update the scripts to have the most-used option as default. Thanks.