Inspired by the "are you sure you're holding this project right way up?" and a few more discussions, I just wanted to remember the original planescape boxed set. Inside, there was no detailed description of baator, no detailed map of faction headquarters, no descriptions of separate realms etc. It had four books with minimal description and it was more then enough for starters. Would it be enough for urban version?
Since times have changed and player/DM line is almost invisible (any player could get talked into DMing), dividing player and DM guides makes no sense and the set could be a little different. My proposition is:
-Clueless guide to the multiverse (fluff on planar science for people unfamiliar with the planes - primes, proxies, gates, astral cords, spels, feats, elements, planar conditions, cyberspace etc)
-Atlas of the planes (nobody here's a mapmaker so it might take some renaming, but here you could put all the possible planes, layers, realms, towns, roads, elevators, staircases, highways etc)
-Monstruos compendium (if enough monsters are collected)
-Whered'ya wanna go today, berk? (introductory adventure for level 3 heroes)
After you have draft versions of these, we could all try a few games and see what fits into the setting in practice 
Still, the way things are done now, some places are unmentioned and some are so detailed, i could walk through them and know where to step without stumbling.
If you had some basic info in one place, it would be easier to run as an adventure or two. It's nice to think up stuff, but in the end, it would be nice to make it playable for rookies and people with short attention spans 
That's part of why we're working on Sigil right now, because it's the one place that almost every PS (and by extension, UPS) campaign makes use of.
Definitely, sigil and outlands are the most important
I don't have much to say on the other end of things - that's more Ulden Throatbane's end of things if he wants to prioritize what our volunteers come out with along those lines. (Prioritizing any volunteer effort can be a paaaain.
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I will say this - I'm beyond unlikely to make a wiki for it. Part of the reason we want to have a downloadable PDF of this is because that makes it something we can much more easily advertise and market outside of the boards. It becomes a concrete 'thing' that we can get people to play who haven't been playing it before. It's hard to get people playing based on a wiki.
In the end, assuming that the project is done in a prioritized way - after what you would call the 'box set' information has been put together, the rest of it will follow - and probably just be additional chapters to the original.