The pro-model Canons (1D, 1Ds, 1DMkII, etc..) have a "quiet" mode you can put them in. It helps dampen the mirror slap a bit, but it's not silent. I've used it a couple of times just to try it out, but I normally don't need that functionality so I haven't looked at it too closely.
You want really quiet, check out a rangefinder. No mirror to slap around, and many have cloth shutters. Much quieter than SLR, but I don't know of any that are digital right now.
Of course, point 'n' shoot digicams are quiet also...no mirror or shutter to slap around (electronic shutter).