I was under the impression once we saw him, uh, well.

[hider=Dr Who Spoilers] 
When we found out he was the Face of Boe, and than when we see the Face of Boe die, I figured time-physics or whatever science the show applied to Time Lords, he'd be unable to travel to a point where he could visit Jack Harkness/Face of Boe. Of course, I could be wrong, and there could be a future episode with Jack Harkness, but as for the Face of Boe i'd say we've seen the last of him. [/hider]