After I was approved and got my first sticker, the CBP officer put it on the inside back cover of my passport. I recently renewed my passport, so I had to bring it into a GE Enrollment center for them to give me a new sticker. This time, the officer put it on the outside (like the one in FriendlySkies' picture.

(On a side note: Just passed through ORD yesterday. Again, very glad to have GE. Our flight arrived just after other widebody based on the number of people in line. Went straight to the kiosks on the side wall and got my slip within 30 seconds. Headed straight for the exit where the officer took my slip, asked to see my passport to check for the sticker, then sent me along my way. ^)