Mine usually beeps once.
Sometimes however, it will seem to be taking a longer than usual time to shut down... and then it'll beep three times before turning off.
I'd always just assumed it was busy doing something in the background, and waited a while before forcing a shutdown.
Does seem to depend on what I was doing before, like if I drop straight out of a game, and try to shut down immediately, it may take longer and beep three times, as opposed to just using the XMB, where it shuts down quickly with 1 beep.
V1N.