Bumping this up again, because i've just seen some more food for thought.
The "Dimension Jumper" spell specifically mentions:
DIMENSION JUMPER
[...]
If you don't end your teleport on a solid surface, you fall immediately.
Ironically, it's the very last sentence of the spell's description, apparently placed there for the single purpose of making us wonder again!
Anyway, this seems to imply that an empty square in the air is a valid target for someone using a "Conjuration [Teleportation]" spell, at least for short range, regardless if it's an "offensive" or "defensive" spell... provided, of course:
a) the caster has line of sight to the target square (avoids teleporting someone into a wall or into the ground)
b) line of effect to the target square (same as above)
c) beats the resistance of the target creature (SR and/or a probable Will save)
This would at least explain the tactics mentioned in the Duskblade's handbook and by Phaedrusxy (the "dropping creatures off a cliff" thing).
Only certain teleportation spells require that the location be within LoS/LoE. Else you couldn't teleport around corners or past doors, and the long-distance teleports would be largely useless.
