Do the horns and tail of a Tiefling have any mechanical effect on the game, or are they merely for show? If they don't have any game effect, then you could describe a Tiefling anyway you liked and it wouldn't matter (i.e. MY Tiefling has no tail or horns, but instead has red skin and fangs, etc...) Maybe the Tieflings shown so far just represent what an "average" Tiefling looks like, and there are others that depart from the norm...
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No game effect, and there is actually a picture in one of the new Dragons that shows 4e Tieflings with a wide variety of apearences.
There's a thread about the picture here somewhere...
<<Goes and looks>>
Ah, here we go. "Non-standard tieflings in the new Dragon article"