I wouldn't be so sure about that
Sure they put in a lot of effort to make it appear like a secure system. However your mail still needs to go through a backend server running with high privileges. It did have security critical bugs in the past, and since it needs to decode all e-mail it's likely it will have them again in the future.
Blackberry's chance would be to open up their protocols so others could also write backend servers, and ideally to provide a greatly simplified open source version of their operating system. The Pentagon might in fact have both internally.
So currently I wouldn't trust Blackberry from a security standpoint, particularly since they had an issue with sending IMAP and POP3 passwords to a server in Canada.