OOPS! I lost my DOMAIN on the hard drive!

Home/Library/Application Support/iWeb is where the Domain.sites file is located.
iWeb can't be used to import already published files.

Unfortunately your "domain" is not on the Apple server.
iWeb stores all your website information into a file called domain.sites in iWeb '06 and domain.sites2 in iWeb '08. This is stored in the iWeb folder on your hard drive that QuickTimeKirk directed to you.
As soon as you publish your website you can forget the word "domain".
Your website is uploaded to .Mac as a bunch of folders containing all your site information.
This action is NOT reversible. You cannot convert your "Site" files back to a "domain" package.
If a spotlight search does not reveal your "domain.sites" file then you have lost it and there's no way back.
When you launch iWeb '08 and it cannot find a domain.sites or domain.sites2 file, it will ask permission to create a new one. Select this option and build your new site.
Now that you have suffered the agony of losing your website, learn from the experience and make regular backup copies of your new domain.sites2 file(s) and save them, at least, to a different location on your hard drive if not to an external drive and/or DVD.
You can, of course, make a backup copy of your domain.sites2 file, put it in a folder and upload it to .Mac as a "safe" backup. If you do this, then you can say that your "domain" is on the Apple server.
Anybody with a .Mac account can use the free "Backup" software and set it to incrementally backup their iWeb folder on a regular basis using the "custom" option.

If a spotlight search does not reveal your "domain.sites" file then you have lost it and there's no way back.

Not necessarily.

Many people have been using Domain.sites file management utilities, the latest of which is MultiSites for iWeb. This utility will actually move your Domain.sites file from it's default location into a new directory in your Documents folder, and will "cloak" it under a different file name. In this case, Spotlight will not find it, but it will still be on the hard drive.

Not trying to be combative or argumentative, but this is worth pointing out, as this has caused some unnecessary anquish for some. See this thread in the '06 Forum:

http://discussions.apple.com/thread.jspa?threadID=1109921&tstart=15

As Old Toad points out in the thread, it's probably best to stay away from any utility which moves the Domain.sites file from it's default location and stores it in this manner. (MultiSites latest version does claim to offer an alternative location for the file).

-Mark

Thanks for pointing this out. I tried out the latest version of MultiSites for iWeb a couple of weeks ago and, due to my personal opinion of it, decide to trash it with AppZapper. I had forgotten that it is one of these applications that dumps a folder into "documents" which I tend only to use for "personal" documents. Fortunately, AppZapper didn't forget!
Prior to iWeb '08 I used iWebsites. Apart from working very well and storing a directory of your sites right next to the iWeb folder in application support, this application had a very useful "merge" feature and is also free!
Until such times as the author of iWebsites can get time to update the application and restore the "merge' function, I have reverted to storing the domain.sites2 files for my various websites in separate folders.