When opening iWeb on another computer that is connected to .Mac, you will be presented with a blank iWeb site to start from scratch, and you cannot open a previously-created iWeb site to edit. The file that keeps track of this all is ~/Library/Application Support/iWeb/Domain.sites
I quickly fixed my setup so that I can use two Macs to edit iWeb without copying a file. I thought of first using iDisk, but my Domain.sites package was already 23MB and the copy was slow. So I decided to use a flash thumb drive. I moved the Domain.sites file to a flash drive that I always carry around with me. Then I deleted the file from the source folder.
I then opened Terminal and typed these two commands:
$ cd ~/Library/Application Support/iWeb
$ ln -s /Volumes/ThumbDriveName/Domain.sites Domain.sites
The first changes into the proper iWeb directory and the second creates a link (sort of like an alias in the Finder) between the file on the thumb drive and the location where iWeb expects to find the file. Obviously, replace ThumbDriveName with the actual name of your thumb drive.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060114142358803