I've been giving thought to a running a modern or near-future Planescape game, using my own take on how it would work. Among other things, the setting would be higher magic, but magic and advanced technology are mutually exclusive. I'd be looking for 3-6 players who want to play low level primes who may or may not have had access either magic or technology.
The real question though is system. There are a number of possibilities, but it's no simple choice. These are the options as I see them:
-3.5 D&D: Not happening. I'd not wish a modern game run with regular 3.5 on my worst enemy.
-20 Modern: I've run it a fair bit, and am quite familiar with it, but it is a bit inflexible, and doesn't balance well with high magic without a bit of tweaking. On the other hand, the SRD is free and it's essentially 3.0 D&D.
-3.5 using generic classes from unearthed arcana (SRD available free): It's doable, balances with magic, and it doesn't require any steep learning curve. On the other hand it's rather simple.
-3.5 using Eclipse: codex persona (available free at RPGnow) This is a good system, and extremely flexible, but it's rather fiddly, and a bit more bookwork than the other options. I've run it and liked it.
-True20: This is a hard sell, because it's an entirely different system, but it's such a simple and elegant system that it's not much time or effort to learn. If you don't have the books or PDFs youd have to *obtain* them. True 20 is my favorite system, but, I'm well aware nobody plays it.
-GURPS: I'm joking, don't worry.
You may be able to find some useful stuff amongst the Urban Planescape boards here too. I believe they were running under a d20 Modern base.