I've had 3 associates who frequently use MtGox and SR receive calls from the ATF.
Could this be substantiated?
If the calls were actually letters, then yes. Otherwise who knows or cares. Silk Road is just like any other criminal enterprise: the stupid and/or cocky ones will get caught and the smart ones won't (they're also the ones that will stay out of trouble, but for their vices). So really who cares?