Blackjacking is a technique used to connect internally to a corporate network using a typical BES device. It was announced at DefCon a few years back and is hardly new.
How does this work, and what are the ways to reduce risk from this exploit?
In short... exploiting a blackberry device or BES... to attack your LAN directly. There's several ways of accomplishing it. Email-based exploited PDFs are probably most common, as they can affect the BES server directly or turn the endpoint into a proxy for attacks. What most people don't realize is that a BES server will proxy ALL traffic for any device registered to it. If the BES server is not properly kept in a secure DMZ, you can use the server to attack your network from the inside.
If a handset is exploited (using any means), anything your BES server is attached to can be attacked. Any mail or other communications processed by the BES server could also potentially also exploit the BES server and use it as a proxy.
A blackberry device which is connected to a BES server has a VPN to the network that the BES is connected to. This is often used as a feature, e.g. to deploy internal applications directly to Blackberry handsets. But it means that any application running on the handset potentially has access to the network. So if an attacker can get you to install a malicious app then this is essentially running inside your firewalls.
There are a number of preventative measures that you can take.
I'm not aware of any Blackberry device based exploits in the wild, but I would definitely recommend doing as much of this as possible in your environment.