It is totally reasonable.
What is not reasonable is a player thinking that this is an automatic action with no chance of failure, especially when the player does not mention his intent to jump and teleport...
If you read my previous posts in the thread, you'll see I've said that twice already.
Reasonable? Maybe. But I don't know how you can come to a conclusion that this is the intended rule.
I don't know what the designers were thinking. That's just how
I would rule on it.
Edit: Actually, thinking on it, this is an area where I'm reasonably sure I
do know what they were thinking:
In 3.5e, a character could make a jump such that his Jump check allowed for more squares than he had movement (that is, he had 4 squares of movement, but got 25 or more on the check). In this case, in 3.5e, the character would end his turn in mid-air and finish it at the start of his next turn. (This actually makes perfect sense - it's an artifact of the turn structure that characters move in discrete 30 ft blocks, when in reality that jump would be one continuous movement.)
The problem with this, of course, was that the character would seem to 'hang' in space, motionless, from one turn to another. This caused a number of people problems making sense of the action. So, with 4e, the rule was changed so that you couldn't 'split' a jump in this manner - if you ran out of movement, you fell, regardless of the Athletics check. (From a strict simulation POV, this is wrong, of course, but this is one area where simulation actually hampers the ability of players to make sense of what is going on - odd, no?)
Because I'm
reasonably sure that the rule was written this way to fix that hole in versimilitude, I'm also reasonably sure that the spirit of the rule would tend towards allowing "jump-then-teleport".
(Incidentally, I think there's a better fix, that neither breaks the (loose) simulationism of 3e, nor requires allowing characters to 'hang' in mid-air: do away with strictly fixed movement rates. Allow a character, when running, to make an Athletics check to add a few squares on to his movement rate depending on the check. As an added bonus, this also gives you a built-in chase mechanic, something the system doesn't really have when everyone moves at exactly 6 squares all the time.)