sparkflower.livejournal.comThe fic begins with some random crewman downloading a porn vid and then gets the shock of his life to find it's one of Spock nailing Kirk. After the initial shock, he finds it totally hot (who wouldn't!?), but then the reality of jacking off to thoughts of his superior officers and this is so wrong sets in. He goes to Sulu I think and shows him the video and eventually they take it to Kirk, who was pissed because he wasn't aware that he and Spock were being recorded during that hotel stay in Somewhere. I remember the fic mentions that the video has already been downloaded several hundred (thousand?) times before it was taken offline xD Kirk and Spock eventually film a new, deliberately-made sex tape (this time Kirk nailing Spock), and sends it to the crewmember as a thank you of sorts in the end. (If only all thank yous were this awesome.) That video was personally programmed by Kirk and Spock (so y'know, most secure, unhackable file ever) to be viewable only once before it deleted itself.
I'm almost positive I read this on KS Archive, though it might have been cross-posted elsewhere, who knows.
If you find me this fic, you'll gain my undying love! For what it's worth :'D I'm still sifting, rather hopelessly, through my fic collection in a bid to find this. Hopefully someone can put me out of my misery!
Edit: All my love to chanel_5 for locating it! *happily bounces off to re-read*