(Please excuse me avid HPC readers.This is an old post I resurrected.There have been over 2,000 hits to this question so I thought I'd post this reply again)

Hi there.

My theory about the sword of Gryffindor is that it can be claimed by any Gryffindor under circumstances of Need and Valour. I think Dumbledore , knowing , the sword rightfully belonged at Hogwarts and that Hogwarts would probably be the final battle ground where Nagini would be an obstacle would have caused him to find some way of ensuring the sword would be in the right place at the right time, so that anyone who was not a Gryffindor (e.g Griphook) would loose possession should the need for it's use be required. The sorting hat would then be a sort of portkey, a passage for the swords safe return to Gryffindor hands.

The fact that Neville had the sorting hat on his head and was being tortured would have been the key for the swords return as Neville's thoughts would have been heard by the hat. Neville seeing the snake and having been specifically informed by Harry that it must be killed must have been thinking " I have to kill the snake" and "presto" the hat provided the solution (just like in book 2).

Also, the sword is a highly powerful magical object with the ability to slay ancient magical beasts (Basilisk Book 2).