Currently Being ModeratedNov 13, 2012 2:16 PM (in response to Help Bilbo)
iWeb keeps its data in a file called 'Domain' which by default lives in (user)/Library/Application Support/iWeb and it this which you should back up.
Note that in Lion and Mountain Lion Apple have hidden the user’s Library folder to casual viewers, in an attempt to stop new Mac users messing their machines up. To access it, in the Finder go to the ‘Go’ menu and hold down the Option (Alt) key; the Library folder will appear as a choice.
Currently Being ModeratedNov 13, 2012 4:27 PM (in response to Help Bilbo)
See this page for info about the Domain.sites2 file...
... and this page for info about opening the library folder and a Domain file when using Lion/Mountain Lion...