Just came across an article where it says "Pentagon approves Android device for Department of Defense ...". It is not unknown that BlackBerry OS has been the platform of choice for the Department of Defense for many years thanks to RIM’s robust security. I wonder if that is coming to an end? At this point, it is only Dell Venue, but none the less, this could be the start of Android taking over RIM's dominance when it comes to choice of security. The detail article can be found here: Pentagon approves Android device for Department of Defense, Apple still awaits clearance
The Pentagon is just approving based on it's security specifications, if a device passes then it can be used. I don't think it's necessarily a mandatory shift from RIM. Just an approval for someone to choose to use another device.
Reading the article's blog on this subject you will see a discussion on hacking both android and Apple. Not one word on blackberry being a security risk. I would think the DOD is simply looking for a cheap cell phone for a limited audiance.