Automatic creation of new site?

Have set up WP3.01 with sub-domain mutisites and all looks good – created a couple of sites, added modules, changed themes etc and it works!
What I would love to be able to do is have a new client sign up for a site and then have the new site automatically created (better still – after they have completed a Paypal payment!) – like a mini hosting company. Is there anything available that would let me do this?

Many thanks