To tell or not to tell... that is the question

Definitely a situation we all hope we’ll never be in is discovering that a friend’s partner is doing the dirty without our friend’s knowledge. Do we tell? Or do we – usually rightly – expect that our friend will shoot the messenger?
Anyone who watched Cashmere Mafia the other night will know where I’m coming from. Zoe spots Juliet’s cad of a husband in a public clinch with another woman, and bands together with the rest of their gal-pal group to deliver the news. Cashmere catfight? No, Jules takes it quite well, giving a sad little speech about how she prefers her life to the alternative (single), but later socks it to her hubby by revealing she will be taking one of his friends as a lover. Ouch. OK, so it’s not exactly how most people would like their marriages to operate, but whatever revenge tactics float your boat.
Anyhow, it’s the ‘telling’ that got me. Would you, should you, could you? My friend Amanda says she’d only tell if she saw the evidence with her own eyes. “Rumours about how a friend of a friend saw so-and-so’s boyfriend with someone else wouldn’t cut the mustard,” she says.
Ellen, on the other hand, says she’d prefer to approach the cheating partner and clobber/convince them to ‘fess up. “If they didn't, I’d tell.”
And it’s not always cheating that you’re torn over telling. Spare a thought for this delightful little triangle of deceit I experienced many years ago. I was royally dumped – at my own birthday party no less – by my boyfriend who was planning to go back to his home country to work. Nice timing, I know, but the thing that left more of a bitter taste in my mouth was the fact that my very good friend and flatmate knew of the dumping before I did. My boyfriend had announced his plans to her prior to the fact and asked her to ‘look after me’. At the time, I wanted both to fall under a bus, but over the years I have come to see she, too, was in a conundrum: here I was having a ball; in telling me she risked wrecking my night – especially if he chickened out and never went through with the said dumping. Also, he was a jerk for involving her in the first place. Would I have held my tongue like she did? Who knows. I hope I never have to find out.
Sooooo, gang, spill it: have you ever been in a situation where you witnessed or heard something you really should confess to a friend? Did you? Or when would you?