Here’s the situation:
My best friend thinks that I still hate a mutual acquaintance of ours. The only little detail is we’ve been dating since Valentine’s day. She didn’t want me to tell him until tonight.
I didn’t steal her, and they never dated, so he won’t be mad or anything (just shocked). But I’d still like to break the news to him a little deviously (via email). Got any suggestions?
I couldn’t find a cheesy “surprise!” email greeting card…
Sneak into his apartment when he’s not home and wait for him. When you hear him coming, start having sex. When he walks in, act surprised and tell him you two had decided to put your spat to rest and wanted him to mediate. You both got here before he did, let yourselves in, and one thing let to another, and… well… erm…
Then tell him what really happened. It’ll be hilarious.
I was thinking of a different version of what Homersaid; meet your friend at a bar or restaurant, whatever, have your girlfriend walk by casually; she stops to talk to your friend, you and she start to have a fight, it escalates into a full-fledging yelling match, then suddenly, just like in the movies, you and your girlfriend start kissing passionately and rolling all over the table, throwing drinks off in your wild abandon. That would surprise your friend.
Quickly marry her, and then, when you meet your friend, announce that you and her are getting a divorce.
I don’t advocate casually marrying or divorcing, but your friend would certainly be surprised.
Mess with his head; Act as if you’ve always been together, better still, get as many other people in on it as well and make sure that they are there when he sees you together for the first time, so that when he says, “hey, er, you two are… but I thought…”, everyone else will put on surprise that he didn’t know; “how could you not know? they’ve been going out as long as I’ve known them…”
Take a polaroid of the two of you together, naked (or something compromising) and stick it to his fridge with a magnet.
Ooh, I like this one! 
Just come right out and tell him. Then kick him in the nuts and throw a bucket of icewater over his head. That’ll sure surprise him.