Main Domain To Sub-domain

I am trying to achieve pretty much exactly what google does in blogspot supporting getting your own domain name.

I have sub-domains (eg: john.mydomain.com) that user uses today. Now, if some user decided to get their own domain name (eg: john.com), what do I need to do to so that contect from john.mydomain.com is served under john.com, keeping the browser URL john.com

I believe this is exactly same as what google, wordpress etc does to support your own domain name. As far google, they ask you to enter a CNAME record such as www .john.com pointing to ghs.google.com -- I guess they must be having some code at ghs.google.com that figures out what content to present ?

I can pay to get this working... please help.

Since alot depends on various stuff like your host, what system you are using, the programming language, I would say pay htaccesselite to do it.. it may even be delegated to me!