Manipulate the address field in the browser?

Here's a crazy question. Say I have a script (such as a bulliten board script) that is run by many clients (other websites) off of one location on my web account. Think of it as a service of some kind. Is there anyway to trick the browser into displaying the client's domain rather than my own? I would want it to look as though you've never left their website when in fact you are now on mine. Possible? Much thanks, -Kevin