Sounds like you need to use the detach tool, available in the first third of left toolbar when poly select is active.
If you want it as a separate element on the same object, you can hit ‘detach’ and check off detach as element. If you want it as a new object with a unique name, you can just detach it. Finally, if you want a new object from the selected faces but don’t want to delete the faces from your old object, you can detach as clone without impacting the selected geo at all.
[editline]13th February 2015[/editline]
All that said, your best bet is probably just an inset operation.