Some folks at Planewalker are working on a d20 modern Planescape and as a planescape fan I think its cooler than sliced bread, Pumpernicle bread even, loaded high with pastrami, swiss cheese, crispy lettuce thinly slice onion and spicy mustard. MMMM.
I like the idea of Sigil with all the factions and modern tech (future is a bit like B5 so I'd avoid that) Now the Doomguard really have an Arsenal, the Hardheads cruise the streets in patrol cars, and the Xaositechs... do their thing. In my head I see some of the factions acting like the Mafia and some acting like gangs and even some of them just being loose affiliations of berks or "clubs"
In my Planescape Modern the City of Doors is a combination of the wild streets of New York Hong Kong and London, still grey and grimy, but now filled with Gnoll Pimps, Tiefling Fixers and Bariur teamsters. Heck it has its own Vigilanties.
The Lady of Pain is still around, and still just as scary as ever.
Thinking of some of the planes in a modern way would be cool too. What does Asguard look like? Viking Dwarves in Combat armor with Barrett Light fifties?
Has the Blood War become atomic, Heck yeah it has!
Arcadia looks like a 1950's leave it to beaver episode.
And Mechanus is a cyberpunks dream, its totally the internet of the planes now, welcome to the Mechanus Matrix Mr Anderson!
Some thoughts on the Planes in D20 Modern are also on the Wizards site and they're a cool read
http://boards1.wizards.com/showthread.php?threadid=178643