It's the 1951 classic, The Day The Earth Stood Still, and I really, really hope there isn't some subconscious reason for its recent (and frequent) appearances on my cinematic mind. It's on NETFLIX INSTANT (along with a summary that amuses me greatly):
An envoy from another world warns Earth's people to cease their violent behavior. But panic erupts when a nervous soldier shoots the messenger.
Well-played, Plot Synopsis Guy; well-played. (Also, a related point: Michael Rennie is a fascinating-looking guy, and a huge part of why this film is so successful/eerie. The Scott Derrickson-helmed remake from 2008 -- which I enjoyed a lot more than most -- featured Keanu Reeves in the messenger role. And while that seems like an excellent decision on paper, Reeves is simply not as intrinsically interesting/unusual as Rennie.)