I never realized this and I am surprised that BlackBerry and Research In Motion advertising have not made a big deal out of it. According to this article BlackBerry communications travel over a highly encrypted channel, not the open internet:
"Other smartphones, like the Apple iPhone, are not tied to one e-mail service. In general, that means e-mail to and from the devices mostly travels over the open Internet and can be relatively easily monitored.
But the BlackBerry uses highly encrypted data that is received by wireless carriers’ towers and is immediately routed through a closed, global network operated by the company. To enforce the ban, the carriers will stop forwarding that data." (from New York Times)