You still can, but the fact is that Apple no longer sells iWeb or supports it and has not done so for sometime now. iLife still ships with new Macs but just not with iWeb or iDVD. MobileMe closed at the end of June last year so Apple withdrew iWeb too, but you can still use iWeb if you want to, but you'll just have to publish your site on another host/sever.
If you have installation CD's from an older Mac, then use these and if it has iWeb on it, then you can copy it, but if not, then you are lookig at purchasing either iLife 09 or iLife 11 - both contains iWeb 09 from Amazon.
Once you obtain iWeb 3 (iLife 09 or 11) the following will be of interest to you:
In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
To open your domain file in Lion or Mountain Lion or to switch between multiple domain files Cyclosaurus has provided us with the following script that you can make into an Applescript application with Script Editor. Open Script Editor, copy and paste the script below into Script Editor's window and save as an application.
do shell script "/usr/bin/defaults write com.apple.iWeb iWebDefaultsDocumentPath -boolean no"
tell application "iWeb" to activate
You can download an already compiled version with this link: iWeb Switch Domain.
Just launch the application, find and select the domain file in your Home/Library/Application Support/iWeb folder that you want to open and it will open with iWeb. It modifies the iWeb preference file each time it's launched so one can switch between domain files.
WARNING: iWeb Switch Domain will overwrite an existing Domain.sites2 file if you select to create a new domain in the same folder. So rename your domain files once they've been created to something other than the default name.