Just about everything to do with a modern BlackBerry is designed to be totally secure.
The phone can be protected by a Device Password that, if it is not eenteredcorrectly after 10 tries (even the number of tries is settable) the phone will wipe itself of all user data, clean its memory and reboot with no user data.
You can encrypt the phone's data with a different password as well as using a Device Password. This makes it nearly to the phone and copy meaningful data.
The Backup file to your PC can also be similarly encrypted so that the Backup files cannot be opened and data viewed or copied.
Over the Internet RIM (BlackBerry's makers) have a private network (a VPN) that is both compressed and encrypted using TES (Triple DES) that is just about uncrackable. The Indian government recently shut down the Indian blackberry.net called BIS (BlackBerry Internet Service) because it was unable to crack RIM's encryption.
This multiple aproaapproach each segment or part of the system being highly secure makes it very hard indeed, even for law enforcement agencies to access a well protected and encrypted BlackBerry phone.
In the corporate environment there is a RIM system called BES (BlackBerry Enterprise Network) that is equally secure but also allows the corporate administrator to totally own his employer owned phones on his network. Such phones maye bmaymotely wiped, locked, reloaded. Internet access may be barred or restricted to certain sites and phone calls may be restricted to any phone list or left with free access to all numbers. BES can stop programs being installed or run and security attacks can be reported back to the administrator.
So, now you know. I hope this helps.
I am sure that others will add to this list.