Generic Zone File??

[Joe]

We have a customer  that has over 10,000 clients using his web site.  They 
connect to their own sites by using   clientdomain.com/username as the URL.  
They would like to allow their clients to register their own domain name, 
using the clients DNS server.  That alone would be pretty easy.  The problem 
is they don't want to have to generate a seperate zone file for each domain.  
IE, they want every domain registered to their server to resolve out to their 
web servers ip. I told them that I didn't  think that it was possible, but 
that I would put it on the list to make sure.  Does any one know of a way to 
do this??

Thanks,

Joe