It is a lot harder than it used to be but it depends on the airline (not many are allowing 135 jumpseaters). I know of a few 135 guys, including myself, that have jumpseated recently. I'd figure out where you want to go, and then call the airlines that serve that destination, and ask them what their policy is.
We are now limited to the cabin (fair enough) so I cannot understand why the airlines that used to allow 135 jumpseat are not allowing it after 9/11. Are we more of a threat to a flight than the general flying public?