It cost me hours of searching and experimenting, but it worked. Here’s how to have the web content for http://www.your-domain.xyz/ served up from an existing Apple “Mobile Me” (a/k/a .Mac) account.
Use the DNS controls provided by your domain registrar to create a CNAME for “www” in your domain pointing to “web.me.com.”
Use the Mobile Me “Settings” page (the gear icon); pick “Personal Domain” from the left menu (a green @); click “Add domain”; fill in the domain; and Continue.
Put your content on your iDisk in the folder Web/Sites. You may have to create that folder. You may have trouble creating a new folder in Web and then renaming it to Sites. If so, create a folder “Sites” elsewhere and drag it into “Web” on your iDisk.
UPDATE: This is pretty much moot, since Apple will drop Mobile Me’s web site feature June 30, 2012, with the change to their “iCloud” service. Migration instructions are at Apple Support.