Thanks for all the pointers everyone
I think I'll be ok - I've previously used non-standard bbs on this BES and had no trouble. As someone said, it's just a case of re-activation on the device.
I also take on board what others have said about company policies and the "point" of using a personal BB on a corporate BES. In many ways, I just want a better handset and am willing to fork out a little to get that. I don't plan on using personal email on it, and have no real issues with the company effectively 'controlling' the device. The only difference me owning it makes is that if I leave the company, I take the physical device with me.