I've purchased Celestial Seasonings' Morning Thunder tea off and on over the years, since the days when it was nearly the only bagged tea with yerba maté that you could find in the U.S. I had always vaguely remembered that a bag was supposed to have the caffeine content of a cup of drip coffee, around 100 mg. And a Google search of old Usenet postings shows that it had the reputation of being even more potent, or perhaps nefarious; the FDA and/or one or more states were rumored to have banned it sometime in the early '90s. (I do remember it went missing from the shelves for a while.)
Whatever the case in the old days, it's currently simply a blend of roasted maté and black tea. And according to Celestial Seasonings' website, an 8 oz. cup contains 40 mg of caffeine, significantly less than the 65 mg in a cup of their pure organic black tea.
I'm pretty sure that the original formulation had added caffeine to bring it up to coffee strength. Does anyone know when it changed? (The amusing thing is that some of the recent Usenet postings are still referring to it as if it were a dangerous drug... I wonder if any aging hippies furtively stick it in their shopping carts when no one else is looking, feeling mildly guilty...)