IIRC, BlackBerry phones can be managed remotely. A nice use case for this feature is to shut down and wipe a stolen/lost phone with confidential information on it. I also believe that data (call log, text messages) can be retrieved remotely. I'm going out on a limb here in saying this, so I'll go look for something to cite.

EDIT: BlackBerry Enterprise Service is what I was thinking of. There's a least one tool that you can use with BES, Retain by GWAVA, that will track and store your employee's text message.