Froissart, my friend, you are a genius! I've been pulling my hair out for weeks trying to find a solution for the exact same problem. As you stated, the BES documentation just didn't cut it. I scavenged every related thread I could find on RIM's site, Microsoft's site, and every forum I could get my hands on... yet nothing helped. After making that one simple modification to the permissions via the Management Shell, my activation started so quickly I nearly fell out of my chair! That was the only thing that was needed this entire time! I couldn't believe it!!! Someone should inform RIM about this so they can modify their documentation accordingly.
You're a lifesaver! Two thumbs way up!