Can metal studs support a 64" plasma TV on a swiveling mount?
I do this for a (part of my) living. Steel studs will not provide the support you need with a cantilevered arm. If you want to surface-mount the TV then you can use toggle bolts through a steel stud, but a cantilevered arm WILL fail.

The only appropriate solution is to open up the wall and either replace the steel studs in question with wooden studs, or slide the wooden stud in to the steel stud.