Update: The BBM download site appears to be working again. BlackBerry Help is says you can download BlackBerry Messenger 126.96.36.199 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 188.8.131.52 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 184.108.40.206 will return, later this week, via BlackBerry App World.