This thread helped me a lot with replacing the alternator in my '02 325i. THANKS!!! I thought I'd share some tips from my experience.
You will undoubtly loose/drop a bolt or tool while doing this. It is really easy to remove the cover under the engine and highly recommended so that the bolt or tool fall to the ground instead of trying to fish it out. The cover comes off with 7 one turn screws and 3 plastic rivets, very well designed to remove and re-install easily.
Also, an easy way to lock the belt tensioner to relieve to pressure is to just pull on the belt once you remove the top bolt/pulley of the alternator. This will position the tensioner so that you can place something in the two holes and lock it in position.