Exams and doors

Well, the first term is almost over (I mean, lectures are over).. the exam is on Dec 11.. and I am already scared...

last post for this season... a brain teaser discussed by James Taylor (a prof at Ox, see below post for who he is)... seems like this is also a TV game in UK.... and most asked question in interviews!

There are three doors - one of them contains and if you call correctly you get it all. Other door contains nothing and you fail if you call it wrong...
Step 1: You can call a door as the door containing gold.
Step 2: The conductor gives a clue on the other two doors ("Door X does not contain gold") and he has to speak the truth.
Step 3: You can revise your call.

Big question: after knowing the clue, will you STICK to your door or SWITCH the doors?

lemme give an example...
Step 1: you call door 1
Step 2: Conductor says "Door 2 does not contain gold"
Step: will you STICK to your first door or SWITCH to the third door?

Answer is only one (not the "it depends" type)... and the reason can be given either in real life format or in theoritical (clue - probability)....

Any takers?