I'll admit right away that I haven't had all that much time to explore. I did, however, get a chance to run around Halvung. I came out at Mount Zhayolm and poked around a bit. There are a several (I came across at least 4) "button" type things, kind of like in Quicksand Caves, which don't seem to be attached to anything. I also ran into a locked gate, with a message saying it was locked, with a keyhole that looked rather simple. Another door I found it let me through, and talking to a portal-ish type thing transported me back to Al Zahbi.
There is a second part of the map (both the mountain and Halvung) that I think is only accessible through this gate. Also, the map posted somewhere else earlier that said Halvung at the top, was rather simple, and people thought it would be a Dragon Aery-esque area for Cerberus. That's not listed on ffxi-atlas.com and I couldn't find a similar looking area exploring the place.
Additionally, there were a few seemingly arbitrary ???s in Halvung, I found at least 3 that said There is nothing out of the ordinary. Based on this, I don't think Cerberus is a force popped NM. In fact, I think he is up on most servers right now, waiting for someone to find and kill it.
No one thought SE would make finding him easy, did they? ;p Now let's get to work / speculating. I'm heading back to Halvung to look around again. (Fucking Mamool Ja killed me)