I would also like to know this - my SSD drive that my Windows runs from is only small and I don't want it filled with the Apps etc, I like to put everything else onto my secondary "main" drive.
I read that the best way to install Windows 8 apps on another drive besides drive C is to move the Users folder to that desired drive. However, you have to do it the correct way. You can't simply drag the Users folder to the new drive.