I looked a bit further into this issue particularly after the recent Blackberry e-mail outage and it appears one of the major reasons companies love the Blackberry service is the absolute control they can wield over their company network of Blackberry mobile phones.
They certainly can set up all kind of usage filters to the internet browsing of company phones, and read all company related e-mail sent through staff members devices.
Time and again the discussion boards offer only one direct simple solution:
Carry your Blackberry for work purposes alone and use a second device for personal calls, texts, e-mails and internet browsing.
The major outage was caused because their system requires all traffic through their devices passes through the one central server system hosted here in the UK - no matter what the content or geographic nature of the call - desk to desk in the USA? Still through the UK central server.
It is this "our system alone" that enables them to offer such a high degree of encryption to have international governments protesting they cannot intercept or hack content sent via Blackberry - but the "our system alone" philosophy also enables them to provide company phone network administrators such a high degree of access and control over phones they provide their staff.
Basically, if you have a Blackberry provided by your employer they have as much access and control over your phone content and it's internet browsing capacity as if you were sat at a company PC within one of their offices.