It's no problem mounting a shelf to steel studs, as long as you mount to at least 2! For the Plasma scrreens we removed the sheetrock and monted a piece of plywood to the surface of the metal studs. This gives you the flexibility of covering a greater area for weight dispersion and allows you to center the shelf and speaker easier.

I also found this
http://www.av-mounting-solutions.com/spkbrac/bt14.htm
Jim