Though I can't speak for the atmosphere in the stadium, it was in fact friendly with the group I watched with, even as the lone American in the group. Crab chips were added to the mix of game time snacks and I enjoyed my Mexican beer out of an Orioles frosty mug (Baltimore pride!), which didn't hurt either. Maybe the extra US (Baltimore) spirit helped, no?
And just a note for those of you that, like me, weren't previously aware, the Olympic team and National team are not one in the same. (Didn't Mexico just win gold in soccer? Yes. Was it the same team? No.) The Olympic soccer team is under-23, with only 3 players over the age limit allowed on the list of players for the finals competition.