To add to the above, see http://btsc.webapps.blackberry.com/b...ListHelperImpl
The article describes the security situation for data in transit given the various endpoints. At one time with BBOS and BBM on BlackBerry handsets BBM messages on BIS were "scrambled" and the key was global (same for all). Now that is not strictly true.
From what I understand BBM is as secure or more secure than any competing applications of the same type and intended use. However, I'm by no means an expert.
The BlackBerry logo status icon indicates the device is connected to the BlackBerry network. If not connected then not communicating with Blackberry services.