Definitely still in the "learning" phase of LED's, dimmers and CFL's. But I've done a lot of research, and actual testing during my kitchen remodel. I've found that even a dimmer that is marked as an LED/CFL dimmer may produce a hum if you're lights aren't up to the challenge. (Always find the weakest link, right?)
So, I installed four Cree LR6 canned LED's in my kitchen. I was missing the fourth Cree at first, so we put in a CFL. When we went to use the dimmer there was a loud and distinct hum. I finally unscrewed the CFL and tried the dimmer, and no hum. The CFL obviously wasn't a dimmable CFL.
But, I don't think the dimmers are perfect either. After "solving" the original hum, I carried a handheld radio into the kitchen while the overhead lights were on, and although it wasn't loud, you could hear a buzz when the radio went through the dimmer "field."