Jeff,
You have several alternatives here.
One, as you mentioned above, is to do the bumper antenna installation. Intuitively, though, it just doesn't seem to me that putting the antenna in the bumper is the optimal location. If you go that route, these instructions give you the retrofit part number and the installation tips -
http://www.bmwmotorsports.org/pdf/BM...-%20Aerial.pdf
A second, again as you mentioned above, is to go the shark fin route. That would certainly look unique on the X5. Not as stealth, but it might be kind of cool.
A third option might be to install a second antenna up in the rear spoiler. I'm betting that there is enough room up there to accomodate a second one. I don't have the part number for that aerial, but I bet Vaheh from Crevier could help with that.
Finally, just a flyer here, but is it possible to get a Y-cable (Radio Shack?) and split the existing antenna to both the Assist module AND the Bluetooth module? Just a thought....
Good luck and let us know what you end up doing.
Jeff U.