I am not an authority on this, but based on my personal experience, it varies on who is your assigned advisor (or so I think). When I first applied, my rejection came via e-mail and snail mail. When I applied the second time round, and heard nothing, I simply assumed I was rejected. Turns out I was waitlisted and I was informed via snail mail, but nobody in the department bothered to inform/contact me. I figured that was because the guy I was assigned to work with is pretty out-of-touch with technology. But other friends working with one of the more on-the-ball profs got their acceptances via a prompt personal email from the prof.