Use MobileME to host multiple domain names
Each account only allows one domain name to be listed. To get full separation, you will need either a family pack (for 5 accounts) or multiple main accounts on MobileMe.

Since iWeb does allow multiple sites, perhaps you could use forwarding or redirect for the subsequent custom domains. Making a CNAME pointing www to your mobile me entry point is the cleanest way to pass traffic, but there are other ways if you don't mind the hassle and exposing your mobile me hosting when the redirect shows the URL where you are hosting.

Most web-savvy people can tell a domain is hosted from mobile me even if you use the "offical" one domain per account method - it just is easier to keep your mobile me name from the address bar of most web browsers that way.