Yep, returns to the sump via gravity. You do NOT want the return at the bottom, if it is then the first time you have a power outage your entire tank will drain into the sump killing everything in your tank, overflowing the sump, flooding your house, destroying your carpet, your PS3 and Xbox and making your house smell like monkey butt for a month.
As long as you have a reasonable amount of circulation in the tank then you don't have to worry about where the outlet and intake are to make sure it circulates everything.