Since we're talking about urban survival, I'm a little surprised that no one has mentioned data security/backup.
I carry a lot of the stuff mentioned upthread, so I won't repeat any of that. But I also carry a 2 Gig USB flash drive on my key chain. On that, I have encrypted archives of all the data that would give me serious pain if I lost it. This includes my tax and financial files, my address book, and various photographs that would really kill me to lose (mostly of the kids at various ages in their lives).
I realize that not everyone has a huge dependency on digital data (yet), but if you do, stop and imagine for a moment what would happen if your primary hard drive and all your backups went up in flames for whatever reason. What data would have you up in the middle of the night sweating because you lost it?
I actually lost all my critical data due to a hard drive crash once, and the fall-out from that was not fun. So now I make sure to have encrypted archives of all of this on my key chain (I also have a copy on a 2 gig SD card in my cell phone -- redundancy is a good thing).
Truthfully, except for the occasional mugging, the most likely disaster scenario most urban/suburban dwellers need to prepare for is a house fire, followed by the natural disaster of your choice knocking your house down. If you think about it, most of the stuff you own can be replaced (eventually) but once lost some data can never be gotten back again.
2 gig flash drives are now reasonably inexpensive. But if you decide to add one to your EDC, do make sure to encrypt your archives just in case you lose the flash drive for any reason. I currently use AES256 symmetric encryption and
gpg to perform the encryption, but any symmetric encryption algorithm that you pick should be enough to keep street scum from digging through your files. I also keep a copy of gpg on my flash drive (both Windows and Mac OS X installers) so that I don't have to scrounge around for the program if a disaster strikes and I'm on a non-familiar machine.