Just purchase your domain name and use 'domain forwarding' to direct the domain name towards your published iWeb site.
Easy!
David
You can also publish your site to a folder and use an FTP client to transfer your iweb pages to a domain. You obviously have to buy the domain and hosting services, I use go daddy for this process and it works great.
Check out www.doteasy.com
Free hosting with 10 e-mail accounts and WEB space.
Domain name registery is standard at about $90 for 5 years or you can get it for just a year as well. They sometimes have specials, I just registered my daughter on a special for $4.95 a year through doteasy. I have used them for nearly 6 years and had great service!