I recently gave a talk on governance at a SharePoint conference and came up with a quick demo about how to use the out of the box workflows for site provisioning. I get asked this question a lot about adding something to the end of an OOB workflow (creating a site, saving a form to a database, etc). I put together something quick and documented it in an article on this blog:
Hope you like it. The basic premise is to use the Content Approval setting of the form library to know when the OOB workflow has completed. And then use an Event Receiver to do the "extra" work. In this case, I use the content type of the form to look up external settings information about where and what type of site to create. Have fun.