My reading suggests that teleports would still only move enemies horizontally.
Since you could teleport someone of a cliff without teleporting them vertically (so I understand your quandry). However, the prohibition against vertical movement is for forced movement only.... which teleportation is not (this is made more clear by the update... i.e. the teleportation rules are different than the forced movement rules, and the newly added teleport save does not require you to go prone).
Also, after two posters specifically asked (here) if you can teleport someone vertically, one of the game editors answered saying "you can".
Perhaps you hadn't noticed, but yesterday about half the forum here disagreed with that particular interpretation
I don't frequent this particular thread, but the matter had been previously resolved on the '4e Rules Q&A' forum, and the writers even corroborated that it wasn't forced movement and had no inherent save (hence the need for the fix)... but yeah, the topic came up constantly across the forums.