Actually, I have been putting alot of thought into making metal armor. It would be more durable and realistic than sheet styrene. Also to "BloodyShadow13" your estimate at weight is unreasonable. Who in their right mind would make a costume with 1cm thick metal peices?
Do you know how thick a centimeter is? It's thick enough for you to hit it with a hammer and make a minor dent. Realistically, if I were to make metal armor it would be around 1/4 (sheet metal is around 1/10 cm thick, maybe smaller) cm thick on average, which by your estimate would bring it down to 90lbs and while that is heavy, it's not too heavy considering the punishment it could take. My school backpack gets up to 40lbs on it's own and I have no problem carrying that, add in the optimal use of other materials in key areas I think that the total weight could be brought down even less which would be completely usable.
On top of all this, the armor I would be making to start with would be ODST which isnt even as much as the Master Cheif's, making it a completely reasonable venture. Add that to the fact that I have a forge for melting aluminum and a figurative "ton" of it lying around and I'm pretty sure I could do it so I'm sure someone else could as well, especially if they are better equipped than me.
Sorry for the rant but I really hate it when people go with the flow and bash someone's ideas instead of finding out for themselves whether is could actually be done or not.