Pathauto.module will do this automatically for you. Check out the downloads page. (This also requires that you enable the path module in your admin/module settings.)
BTW, be careful enabling clean urls. It’s good to have two windows of your settings page open when you try this. If you’re config is wrong, or mod_rewrite not working, you will get an error, and often hitting the "back" button will not reload the old page. There are workarounds to dig back in, but I find it easiest just to have the stable settings open in a separate window.