i'm sure some people have already suggested this....but i think the location could be under the tree in the northern stands of the stadium.
i think it will be similar to Sydney's cauldron.....i.e. it will be silver, disc shaped, and rise up from the tree spot vertically into the air.
again, i'm people have mentioned it, that they will not have a cauldron within the stadium, which i think is true, but because there is 1 week between the opening ceremony and the start of the athletics, it will give LOCOG time to re-structure the northern side of the roof in order to hold up a floating cauldron above the stadium.
this, i think, is the most likely scenario.
unless....LOCOG have planed to light the Olympic flame from the centre of the stage....like Vancouver 2010.
i mean nobody really knows what is under there...apart from the shape of the River Thames.