The validity of Torment

I think this is something best left for the DM to decide completely.

Having said that -

How do you define what's canonical or not now, six years after the setting was cancelled? The company that owns Planescape isn't keeping track. The whole idea of worrying about what is or isn't canon seems slightly silly at this point.

I don't think any of the new Black Isle material will appear in any of Planewalker's official products, though, if only because Wizards of the Coast doesn't own it.

I think everything in Torment deserves to be considered canon.

There's probably more than one rogue cranium rat swarm.

There's certainly room for more than one magic item shop in the Cage, and probably more than a thousand. If they had wanted to use A'kin they could have.

The discussion of what is and what is not canon is important, 'though. Given that Planewalker has been given the task of continuing Planescape, and we have the oportunity to create officially sanctioned material (weather an individual piece is accepted or not), we should know these things in case we would like to tie them into a work of ours for the site.

There is hope: karach is statted in Chapter 6. This was invented in Torment (not being a part of the original Planescape Campaign Setting) and so its use in a planewalker product signifies a recognition of that game's amazing-ness and a willingness to incorporate it into this site's setting.

I also hereby plug the Zerth Prestige Class that Mephit James and I created (located in the Rrakkma section--check my sig). It was based on Dak'kon and makes use of the Circle of Zerthimon. Although it is undergoing revision behind the scenes.

It actually was, though it wasn't very prominent. In the original Planescape boxed set, page 51 of the Sigil and Beyond book mentions it.

"Karach, a transmuting metal used in making magical items, is smelted from the chaos stuff that leaks through the ever-changing gate to Limbo. Only the most dedicated smiths can force the stuff to remain stable, so the smelters and workshops of these craftsmen are usually unchanged from visit to visit."

Also see Uncaged: Faces of Sigil, page 124.

The metal wasn't very well defined, though, until Torment. There wasn't anything about the githzerai using it to make swords, or any connection to the zerths, or what it might be good for.

Dak'kon, the old Githzerai Fighter/Mage, a Zerth, had a Karach blade whose abilities enchanced throughout the game depending on his level and his faith in himself and the teachings of Zerthimon.

The Drowned Nations, the Weeping Stone Catacombs and the Dead Nations were the coolest locations I've ever seen in a fantasy CRPG. So much atmosphere and really creepy art design. I love underground stuff like that. Too bad it was such a small area.

Funny, apart from the Twelve Factols Inn I can't recall much being said about Undersigil. It was mostly fleshed out in Torment. I guess the PS designers were shying away from dungeon-type environments.

I 1st touched on Planescape whilst DMing AD&D, some years back & accustomed myself with a few of the Cutters & Sites, layed out in the Guide to Sigil handbook. Some time later I bought a novel called Torment - read it, then bought the game. After hammering the game until it begged me to stop - I found that much of the interaction, laws & gameplay, mirrored, as best it could with the BioWare they had at the time & found that it gave me a greater insight in the world of Planescape, how it worked - albeit simplistically, & after was able to run the complexity of the Planes with an easier reference.
Much of the Sites, Burgs & Cutters are represented in the Sigil/Cage Guide, but as we all know the DM is God & therefore takes as much or little of that as reference, I found it extremely useful.