You really only need iTunes for activating the iPhone, but I'm guessing the concern is what will prevent the users from attaching their iPhone to iTunes at home an loading it up with music. If you block iTunes (or more so the access to iTunes) at the office, there would probably be little risk of a user saving music on their workstation or file servers (there are lots of tools for filtering media files on file servers). Of course there are other issues that can arise from someone installing hardcore rap (or Brian Adams) music on their (actually a corporate asset) iPhone, and possible offending other employees.
I'm really not sure what options you have, as I'm not aware of there being any security policies (I don't believe iPhone has that capability) that you can apply to an iPhone (like a Blackberry/BES solutions). Perhaps Apple or a third party vendor will put something together, but then I'm not sure if that's really a direction Apple wants to go with the iPhone.
I would pursue a well written usage policy with your IT/Corporate security. It's not perfect, but at least you can get an employee to agree (in writing) not to load the phone up with music or unsupported apps under penalty of termination.
Just my 2 cents.