What's an unofficial "anonymous" login? And, whatever it is, what's keeping trolls from creating it?

I believe what tirwhan means here is creating a separate PM username to use for making posts that you wouldn't normally want to be associated with (for whatever reason). For example: everybody knows who merlyn is, and everybody knows his real name (it's in his signature). But lets say that merlyn wanted (for whatever reason) to create an alter-ego and use it to post. Rather than posting as AM, he might create an alternative PM username, let's say something like...um... oh, I don't know... maybe Paco.... for example {grin}

Apart from the (employer-related) example you cite above, the only reason I could think of for doing something like this would be to protect XP, and that's just silly (tm)

Linking trolls with anonymous monks is like saying "See, there's a notorious driver around who often drives too fast around schools. And he's driving a red car. Let's equip all red cars with a siren, so we know he's coming".

An interesting analogy, but a good one. I have to agree :)