Obama's Inevitable Crackberry Withdrawal
They need to be available to the ordinary user, and additional security always makes a device less convenient to use. They also need to be able to install code updates and open web pages, and that introduces some other vulnerabilities. A foreign power that was determined could almost certainly introduce a malicious application into a standard BlackBerry, and once they had, all is lost. (There are technical solutions for this problem in what is called "multilevel secure trusted systems." One commercial example is Solaris 10 with Trusted Extensions. None of the commercial email systems is in any way "multilevel secure.")
Then there are the legal issues: under the Presidential Records Act, any official communications from the President's and Vice President's Office must be retained, at least until the Archivist can confirm whether they must be preserved long term. This has been interpreted in recent years to include everything including the visitor log at the vice president's residence. It would certainly include BlackBerry emails, even private ones. It was just this kind of issue that caused George W Bush to sign off his email completely just before his inauguration.
These really aren't new issues; it's only that they've become important to people outside the security and intelligence community just in the last few years. Clearly, we don't want to risk the president's online identity being stolen; a hacked email account could be a lot more than simply an embarrassment. But the retention requirements for even ordinary businesses have become onerous and expensive. Perhaps, while he's trying to recover from the loss of his BlackBerry, President Obama might ask himself if there isn't a better way to balance the legal requirements for records against the desire to email "How 'bout them Sox!" to a friend.