If you want one for your costume? I think I've found the company that supplied the jetpack! Have a look at http://www.rocketman.org/
Apparently it produce 140db of noise in use.
Don't know if this has been posted yet? But they had a Guy dressed as a Spartan flying with a jet pack in trafalgar square in London today, to mark the launch of Halo Reach.
Video is on youtube