How to sync iWeb with iCloud (requirements: OS X Mountain Lion with GateKeeper off, iWeb 3.0.4 and iCloud with Documents & Data turned on):
1) Go to Finder, click on the 'Go' menu, hold down the Option key and click Library.
2) Navigate to Mobile Documents, you should see a bunch of 'com~Apple' folders.
3) Create a new folder in Mobile Documents, call it 'com~Apple~iWeb' and create a subfolder called 'Documents'.
4) Create an alias of the Documents folder, and place it in the destination of your choice.
5) Download iWebDrop and copy it to the aliased folder.
6) Open a NEW Finder window, follow step 1, navigate to 'Application Support' and find 'iWeb'
7) Drag the Domain file (don't copy/paste) to your Aliased folder.
Your iWeb domain site is now on iCloud.
8) To edit your website, drag the Domain folder onto iWebDrop and it'll fire up iWeb for you.
9) Edit your website and then Publish/Save it. If you don't directly publish your site to the web, then you can use the iWeb folder as a place to host your website. Otherwise, that's it.
To access it from another computer:
10) Follow steps 1, 2, 3 and 8 (plus 6 and 7 if you want to have more than one Domain)
And you're done. You now have iWeb synced on iCloud.