URL with Username or ID Value

[eluser]RMinor[/eluser]
I have a website with user profiles. In the database each profile has an id value, and if the user wants, a username as well (similar to Facebook). How would you guys recommend that I do searching, displaying the profile, etc. if they have a username? I want the default URL display to be the username and then degrade to using the id if no username is available. I thought about doing a hook because to extend the base controller would mean changing each controller (of course there is search and replace). Any suggestions are greatly appreciated.