Skyscraper
Adventurer
In 25 years of playing, i think i've never been in a situation where my PCs or myself as a player were next to lava. I'm looking forward to it though, because i've read so many threads on the topic that i can't wait for all those rules-discussions to finally bloom into something useful 
This being said, i'd like to know how deep the pit is for that PC to be able to cast a teleportation spell while falling. Plus, normally, someone who's pushed into a pit falls and hits the ground during the pusher's turn. So the teleportation which occurs on the fallen PC's turn essentially places the ally at the bottom of the pit IMO. Unless of course the teleportation power is an immediate reaction, but i doubt that's the case.
The question on the power that places an opponent over the pit appears relevant. I believe the creature would be allowed a save as per RAW since teleporting someone into hindering terrain is moving him into hindering terrain. If we rule it that way, it would mean that the target of the power falls prone on the square where it stood before being teleported if it makes its save and the teleportation fails. It's harder to make sense out of this in game-terms. However, still in game-terms, why is someone being forcefully moved allowed a save to fall prone only if he is moved into hindering terrain? Why not have that option all the time? (Say: if the target is being moved in-between four sneak-attacking rogues, wouldn't it want to fall prone outside of that select circle?) So really, the falling prone rule is really a game-balance rule, and it's up to the DM and players to make sense out of it in-game IMO, including for teleportation.
Sky

This being said, i'd like to know how deep the pit is for that PC to be able to cast a teleportation spell while falling. Plus, normally, someone who's pushed into a pit falls and hits the ground during the pusher's turn. So the teleportation which occurs on the fallen PC's turn essentially places the ally at the bottom of the pit IMO. Unless of course the teleportation power is an immediate reaction, but i doubt that's the case.
The question on the power that places an opponent over the pit appears relevant. I believe the creature would be allowed a save as per RAW since teleporting someone into hindering terrain is moving him into hindering terrain. If we rule it that way, it would mean that the target of the power falls prone on the square where it stood before being teleported if it makes its save and the teleportation fails. It's harder to make sense out of this in game-terms. However, still in game-terms, why is someone being forcefully moved allowed a save to fall prone only if he is moved into hindering terrain? Why not have that option all the time? (Say: if the target is being moved in-between four sneak-attacking rogues, wouldn't it want to fall prone outside of that select circle?) So really, the falling prone rule is really a game-balance rule, and it's up to the DM and players to make sense out of it in-game IMO, including for teleportation.
Sky