Using clientname.companyname.com in place of www.companyname.com/app/index.asp?clientid=1234

I have a client with a web site that he needs to allow his clients a more "customized" look and feel.  For example, his clients can log in to his site and access their data at www.myclientssite.com/app/index.aspx?clientid=1234

BUT, he wants to give them a more personalized look (in part because these clients will be offering access to their portion of the web site to their clients.  So what he wants is some AUTOMATED way to myclientsclientname.myclientssite.com to respond synonymously with www.myclientssite.com/app/index.aspx?clientid=1234

Now my understanding of the technology (using ASP.NET and IIS 6) is that the only way to do this would be to give each client a DNS record and then check the header to see which client's data should be accessed.  But is there any other way?  Or any way to script the creation of the DNS record?  (They don't currently manage their own DNS on their server, but they could).

(I'm asking this on several sites as I need to ensure the broadest possible range of answers - but any answer I get that's not from here I'll post back for your information).