Hey, Asylum. It's been awhile.
Anyway, even though it's pretty much accepted that the Watcher is Bahl'al, I thought I'd present one more bit of evidence (although it may have been posted before, I've been gone so long.)
I was rooting through Silvermines today with the Bungie TFL editors, and in the scripting they refer to the watcher's arm as Bahl'al's arm, and I'm fairly sure the object's tag is called Bahl'al's arm as well. If it isn't, it has the 4-letter code that Fear would have assigned to a tag named "Bahl'al's Arm".
-Joy