http://www.timworstall.typepad.com/timworstall/

It would appear that even if it had gonoe bang it wouldn't actually have gone bang but pop.
Unless we're still missing a part of the puzzle there were two major errors in the construction. There was no oxidiser. Secondly, all commercial propane tanks have valves that release the gas if it heats and thus the pressure rises. Might have got a jet of flame, might even have had a propane tank accelerating along the street on such a jet, but a big bang? Nope, just wasn't going to happen.
Yes indeed, it does look like there's one or more people out there hoping to blow people up. Fortunately, they're not, on the current evidence, competent.

 http://www.theregister.co.uk/2007/06/29/more_fear_biscuits_please/

`The device could not have detonated. Not under any circumstances. You see, the terrorist wannabe clown who built it left out a crucial element: an oxidiser. The device was pure pre-teen boy fantasy.

"We'll heat up these propane cylinders with burning petrol, and they'll go off like bombs", boys the world over have remarked with glee. They don't realise that air is a poor oxidiser, and the only "explosion" they will get is when gas pressure inside the cylinders is great enough to burst them. Then the propane will ignite, and a nice fireball will blossom. A fireball, not an explosion.

Oh, the Piccadilly fireball would have blown the car's windows out, and popped its doors open, and sent various bits like mirrors and so forth into the air at velocities possibly fatal to people nearby. It would have looked really cool, that's for sure.

But an explosive event...a detonation? Not in a million years. Sorry lads: you failed car bombing 101; you did not attend a single lecture; you did not even open the textbook.