Update: The BBM download site appears to be working again. BlackBerry Help is says you can download BlackBerry Messenger via the following link: http://www.blackberry.com/select/bbm/download.shtml

It looks like Research in Motion has decided that all BlackBerry users trying to download BlackBerry Messenger simultaneously was not such a great idea after all and has temporarily pulled the update to arguably the best mobile IM client of all time.  Either this was the case, or, BBM was prematurely released.   Rumor has it that will return, later this week, via BlackBerry App World.