The new U.S. president (Barack Obama) is the first man in that post who is an avid computer user. This was manifested throughout his election campaign. But even before he won the election, he was told by the Secret Service that he would be strongly discouraged from using his BlackBerry, or any kind of email. The reason for this is the vulnerability of email and cell phones to eavesdropping, and use by a foreign country for espionage.
President Obama took care of these warning by simply ordering the intelligence agencies to make his Internet access secure enough so he can continue using his BlackBerry. The details of how this will happen were not announced, and may not be for a long time. But it probably involves high-grade encryption, and perhaps local network enhancements.
Meanwhile, email, Blackberries and cell phones are widely used throughout the U.S. government. There are restrictions on discussing certain levels of classified material on devices connected to the Internet, or cell phones. What American intelligence officials acknowledge is that foreign hackers (government or otherwise) can, if they want, get at any of these communications. It can be very difficult, and dangerous.
The Barack BlackBerry also presents some opportunities, as well as risks. Knowing that this one BlackBerry will be subject to a lot of hacker attention, the NSA can set a trap to detect and identify those hackers that get close. That would be very useful information, and such traps have been used successfully in the past. High level hackers know of this danger. But even taking great care to get past the traps, there is more risk when you try to hack into the presidential BlackBerry.