As a minimum, these are just a few items that I would carry for a universal setup. I actually carry most of this stuff anyway as a basic setup, and just add different items depending if I'm going to go hiking or go on a road trip.

Multitool (more versatile than a knife)
Large Garbage bags (could be used as shelter, water collection, makeshift poncho, etc)
Bandana (for first aid, cleaning, headband, etc)
Flashlight (no, make that a headlamp)
Lighter
Dental floss (for string)
Duct tape
cellphone
very basic first Aid kit
Small button compass (very useful in the forest or coming out of a subway)
extra clothing (probably a windbreaker or sweater)
TP (useful no matter where you are!)
Lip Balm (could be used as a firestarter, sunscreen, or for lip balm)
Snacks
Extra Money(I could improvise anything in the city with enough cash <img src="/images/graemlins/grin.gif" alt="" />, or could be used as a firestarter <img src="/images/graemlins/frown.gif" alt="" />)

These are things I chose that could be used in both situations, whether in the city or the forest. I know a lot of stuff isn't ideal, but I think the point is to have something that is general works for either one?

There are a lot of things I would have listed, but the reason I didn't include them were because they would be of limited to no use in one of the environments. For example, I listed a multi tool instead of a fixed blade knife because I would rather have to use my multitool as a knife than use a knife as a poor substitue for pliers or screwdriver. I also left out a dedicated shelter because I see that as being more specific for the wilderness, same thing with an ax or saw. Similarly, i dont' think cooking gear would have much use in the city. I know with a knife (and enough time) I could probably improvise fire (replace lighter) or light (replace flashlight), but I'd rather not. I think those two items are important enough in either situation to have their own stuff.

Also, I didn't include any type of water purification, because if it's only for 72 hours, I would risk the chance of getting sick in the wilderness, you probably aren't gonna die from drinking polluted water. If the water is polluted in the city, a filter isn't going to be of much help anyway.

I'd like to see what everyone else thinks. One thing I'm not clear on, are you talking about a normal urban environment, or like a city after a natural disaster type environment? If that's the case, then most of the stuff becomes geared torwards wilderness equipment.

BTW Tmpx, you know what your name sounds like if you try and say it outloud?