For those of you who haven't been there already, By The Tree is located... wait for it... by The Tree. The Tree, of course, is a Beijing institution and the source of all the great pizza.
The street address is 43 Sanlitun Houjie,. It's down a small alley, about 100 meters west of Sanlitun North Bar Street. You'll see a set of stairs -- the bar on the second floor, above Smugglers.
Click on the map for a larger view and detailed directions in English and Chinese.
See you there soon!