If you are using Apache HTTP Server they have some nice URL rewrite stuff. Of course you will still need to tell rails what controller you want. I'm new to Rails though so maybe there is a Rails specific way to do this. My personal bias is towards letting Apache take care of any massive amount of URL rewriting.
This got me to for a while. The method you are looking for is underscore. It takes a string as a parameter and out puts it like you want it. After that, you can parse how you need to search in the database.
HTH, Richard