Two items of note...
Rps: the existing iWeb on your hard drive will not be (cannot be) deleted by Apple during one of its updates, so no worries. You can continue to use iWeb to author websites and upload via FTP, no problem. However -given that the web keeps evolving and iWeb's output will not, we need to explore options for moving to another app, such as the aforementioned RapidWeaver, or Sandvox, etc.
For those of us who still need to run iWeb under Lion, and if you have multiple domain sites, iWeb now behaves a little differently (no matter which domain file you double-click on to open, iWeb stubbornly refuses to open anything other than the last file it worked on).
This application is an iWeb launcher, that allows you to pick the domain file on launch (Multiple Domain iWeb
You can also go into the /Preferences/ folder inside the (now-hidden) Library folder, find the "com.apple.iWeb.plist" file, and do a two-step process:
(i) if you have XCode installed, double-click on the file and it will open in XCode. You need to change the "iWebDefaultsDocumentPath" to "Boolean", check "NO" and save. Then,
(ii) do a Get Info, and lock it - thus preventing iWeb from writing its preferred Domain file path, and iWeb will always ask you which domain file you want to use on startup.
Who knows what will happen with updates to Lion... this workaround may suddenly stop working. But for now, it does the trick.