Well, if we wanted to come up with some kind of explanation for the Jedi in peasant robes thing, we could say that it has to do with their rejection of wealth and power as virtues. A bit like how Yoda surprises us in Empire by being a great warrior, it could be that the Jedi believe that it is the small and seemingly powerless - in this case, peasants - who represent the sacred and honourable. It's the sort of thinking you can find in Christianity, Greek Cynicism, Buddhism, strands of Hinduism and many other faiths.

The trouble with the prequels (as mentioned above) is that these supposedly humble Jedi occupy a massive and decidedly gaudy looking temple. Then again, it's not as if this contradiction doesn't exist in the real world also, what with treasures of the Vatican and the many sumptuous Buddhist temples which exist.

I think Bingowings' idea works. In times of peace, the Jedi dress in simple, modest, peasant clothing, representative of their humility, but if a Jedi enrols in the military or decides independently to enter a conflict, s/he'll wear the appropriate clothing and armour.