Migrating from one computer to another is always a pain. There's a great tool built into Macs called migration assistant. It's primary function is to help you move everything from an old computer to a new computer. (I do mean everything!) Sometimes, though, you just need to move a few things- perhaps an iPhoto library, or an iWeb site. Migration Assistant may not be the best tool to accomplish that.
According to iWeb Help, to move an iWeb site, you need to look for the file: Library/Application/Support/iWeb/ in your user folder. I haven't found the file Library/Application. I did find Library/Application Support/iWeb and the domain file was there. On the Apple Support web page on the same topic, it did refer to file I found, so I suspect the Help has an error in it.
I also suspect not only will this file need to be moved, but the actual files will need to be moved as well, if you are publishing to a folder rather than a .Mac account. I'll be working on this with Beth Tovi, at King
Springs Elementary soon, so watch for updates on this.
iPhoto looks to be less of a problem, in that simply copying the iPhoto library to a new computer should do the trick. I expect iTunes, because of Digital Rights Management, will be much more of a problem.