I've heard from a couple of places and my letters (except the Dean's letter) have been in for at least a month now. I wouldn't sweat it though, i don't think we'll really hear from most schools until Nov.

in most programs they wait till November 1 (dean's letters release day) and start sending invitations in first 2 week of November
..at least in my program they print out applications for Selection Committee review ONLY after November 1

we all (i suspect) checking emails 100 times a day. IMO it happens because of "Las Vegas effect"...remember from Psychology - variable amount + variable ratio -are strongest reinforcements... - you do not know when and how much so you keep pushing that button