After continued headaches with focus follow preformance issues (delay on changover, momentary loss of mouse pointer, frame rate drops) using the in-built Keyclone function I was using Octopus just to do this job. This is far from ideal.

I little research turned up the hidden X-Mouse functionality built into vista, a quick registry edit and you have full focus-follows-mouse over your entire operating system and it works beautifully over multiple sessions of WoW.

Details are here:
Registry Edit

If you don't want the full X-Mouse changes (includes copy/paste changes and a few other tweaks) you can just add "1" to the key to get just focus-follows-mouse.