I admit I'm actually not a big fan of this word, but since Planescape still is a book with seven seals to me and my only access to the feel of the setting is the PC game (besides spending a big bunch of money for goode olde game accessories or spending hours browsing the web filtering fan-made stuff from official one*), I hope it helps me to get behind the dark, if I find out how much of the fluff in the game is official Planescape material and how mach has been made up.
The point is that I always get the impression that this setting is too big and I have to know every single bit, or risk making nonsense that kills the purpose of the game, or create a place that is too plain or too strange and violates some ingame logic. Even FR with its myriads of novels ( that I don't read 
) seems to make it easier to put your very own stuff in it.
So, as an example, in the PC game we have an old veteran who sits in the Smoldering Corpse bar and tells you that he has been in the Blood War, the infernal War of Lies on Terras, the Black Centuries War, the Three-Planes War and many others, even the Harmonium War of Liberation.
Besides of the Blood War I've never heard of any of these wars. So... official, or made up (more examples are welcome)? 
*Nothing against ideas of fans, of course, but it's a bit like judging a person you don't know yourself by the reports of a third one, IMHO.
) So if the game's programmers were the DM, she would have invented these events in either case, right? 
) but I can add one titbit:
Sorry to resurrect a thread that has fallen off the first page, but truly this one shouldn't have fallen off that page. At least not without any response at all. So I'm not sorry?
The veteran planewalker from Torment (was his name Cadris or something?) is, as far as I know, making up all those things. I don't even remember them, and I feel like I'm reading those names for the first time when you write them. With the definite and pretty obvious exception of the Blood War. I, too, have a personal distaste for the word "canon," but the general rule is that stuff which Torment made up isn't automatically a part of the game, but some of it is so cool that it deserves to be (karach-wielding zerth warriors comes to mind).
Calmar, I hope you see this thread again, because you shouldn't feel intimidated by the amount of Planescape content out there. It's really just the result of very creative people who love the setting. However, in the past year a number of adventures have been written up trying to help initiate newcomers so that they can get Planescape off the ground and running. Armory99 led the effort to write an amazing adventure called "Desire and the Dead" and others have made their own introductory adventures to varying degrees of completeness. Maybe someone else can post here to point some of them out. Planescape may be complicated, but it's not impossible to employ the "narrow focus" campaign model and only expand outward (i.e. have to learn it all) at the rate that the campaign dictates.