Clean URLS and the Routes config file

[eluser]amosmos[/eluser]
this seems to interest a lot of users, and i would be very glad to hear some experts opinion on how to achieve this...

i know how to route everything to a specific controller with the route[':any'] method, but would like to know what's the best way to do it when you only want some urls to go there.

what i mean is that i want something that does this:
1. if there is a controller that fits the name in the 'controller' segment of the url, go to it.
2. if there isn't, go to another, specified, controller. in this controller i can put the check for the slug and either show the right page, or show a 404.

thanks,
amos