Simple user blog creation on wp-signup.php

Is There a way to simplfy the blog creation for users?
I have the setting: signed up users can create blogs.
The blog creation process is typing: URL, site name, and description.
I want the URL to be exactly the same as site name. This means that the URL could be automatical created from the title(using only smal letters and numbers like this:
Can annyone hjelp me with a code hack that makes the URL based on user inputing site title?
I have seen other sites that lets users create blogs from title only...