I just recently finished running a mystery campaign based around a detective agency in Sigil. The format was a number of oneshots, with a few longer arcs. I post my campaign notes here, in case anyone else might get some use out of them. The mechanical aspects are designed for my 7thscape system conversion, so adapting to D&D will require some restatting. Hopefully the intentions will be clear though.
Episode 1: The Treasure Within
Help a woman find her lost map and save her Bleaker brother.
http://netfiles.uiuc.edu/ngutten2/www/the_treasure_within.pdf
Episode 2: Mirrormage
Help a Xaositect find his lost reflection, in exchange for a true lie and a lost found.
http://netfiles.uiuc.edu/ngutten2/www/mirrormage.pdf
Episode 3: When the World Wants you Dead
Why would a promising young Cipher kill a visitor to the Gymnasium all of a sudden? Can be used as the start of a campaign arc that leads to Bytopia, a secret black market beneath Sigil, and eventually to Shra'kt'lor.
http://netfiles.uiuc.edu/ngutten2/www/when_the_world_wants_you_dead_v2.pdf
The encoded ledger: http://netfiles.uiuc.edu/ngutten2/www/guvner_form.pdf
Episode 4: Bytopia
When my group followed this line, they basically just had a number of personal reasons for their characters to go to Bytopia rather than a specific mystery (some notes on this). However, it links back to the Githyanki in Episode 3. I've used a bit of the Planes-Militant adventure in Planes of Order for this, but it takes on a bit of a different character. Replace 'Gary' with an old estranged friend of one of the PCs for best effect.
The 108 Ideograms thing was a fun sidequest for one of the PCs who had a book store. It has little to do with anything else here though, but you could replace the author with any classic 'hermit on a mountain' the party needs to seek.
http://netfiles.uiuc.edu/ngutten2/www/bytopia.pdf
Episode 5: The Fall of Derinkuyu
I took the idea of Derinkuyu-under-Sigil from http://www.geocities.com/SouthBeach/1482/ps/ligis.html
At this point, there were a lot of party-specific things going on, so you will have to mix and match the adventure fragments for your own groups. One of the PCs (Azora) defected to the Planes-Militant and had his own side-path to finally get to Derinkuyu, dealing with some cranium rats and gnomish burrows along the way. Saer ended up being the point of introduction of the 'bad stuff is happening beneath Sigil' plotline, but it could easily come through any underworld contacts the party has made up to this point.
If all goes well, by the end of this episode, the PCs will have prevented the fall of Shra'kt'lor. So, you've left canon at this point :)
http://netfiles.uiuc.edu/ngutten2/www/fall_of_derinkuyu.pdf
Episode 6: The Essence of Flavor
A parody of Iron Chef. Chefs from around the planes compete for the position of Head Chef of the Celestial Bureaucracy, and someone's secret ingredient has been stolen. Also features an Athar plot against the life of Yen-Wang-Yeh, Judge of the Dead (which succeeded in my campaign and led to all the newly dead of that pantheon showing up as ghosts in Sigil).
http://netfiles.uiuc.edu/ngutten2/www/the_essence_of_flavor_v2.pdf
Episodes 7-11: Seekers of the Word
This is an Inner Planes adventure, looking for a syllable of one of the Words (like the Last Word...). Of course, the PCs don't know this until about halfway through - they think they're just retrieving a ring for a member of the Godsmen.
http://netfiles.uiuc.edu/ngutten2/www/seekers_of_the_word_v2.pdf
Episodes 12-13: City of Ghosts
This follows from the 'Yeng Wah Yeh has been poisoned' plotline. If that didn't happen, this adventure won't make sense. The culmination of this mini-arc is that one of the PCs may gain the power to judge souls and send them to their rightful (in their eyes) resting place.
http://netfiles.uiuc.edu/ngutten2/www/city_of_ghosts_v2.pdf
Episode 14: The Plane of Mineral
This adventure lets the PCs attempt to free slaves from the grip of the Dao, and to explore and potentially release an ancient nightmare too dangerous for even the 'loths to let roam free.
http://netfiles.uiuc.edu/ngutten2/www/plane_of_mineral.pdf
Thats about as far as the campaign got. There was another game involving a detailed theft of a demiplane seed from the Jester's Palace, but it never ran.
*COOL* I'm going to have to get started reading these, and see if anyone would want to help make the conversions to make the adventure open to D&D.
Of course - if you've developed your adaption of Planescape to 7th Seas enough to really run it, you know we do host versions of the Planes in other settings, right?