I am curious to know how services such as heroku manage 1000's of virtual hosts - ie if you create a web site/app, and put it up on these services, you get your own virtual host name - foo.heroku.com etc (the same applies to many other sites that have vanity URLs).

I know with various web servers and proxies you can configure as many virtual hosts as you want - but there must be some upper limit to this ? Do they programmatically add virtual hosts - perhaps spreading the load? Or are there other solutions.