That said, I live in Philadelphia, and I used to live in NYC. I have a license and have driven in both cities, but I don't own a car now and garaged it with my brother when I was in NYC.
Driving in what most people would consider "downtown" is a pain and brings along expenses and difficulties you don't have in suburbs. Like others have mentioned, cheap parking is hard to find, and easy parking is hard to afford.
But what people who haven't actually lived for long periods of time with out a car tend to ignore is that there are times, even in the densest parts of the city, where having access to a car is almost indescribably convenient. Making multiple trips to the grocery store every week because you can only hold X bags in your hands/on your bike/in your granny cart is a pain. Walking a mile to work (like I do) in the rain because all the cabs are taken is a pain. Biking home from the beer distributor with a 30-rack in your messenger bag can be a pain :)
So I think the real task isn't making cars unnecessary for city life, but making cars easily accessible for the time or two a month they'd be convenient. Thankfully for me, innovators like Philly Carshare and Zipcar are already operating in this space. Even Home Depot will rent you a pretty well equipped work truck by the hour.
I don't see what problem the Segway solves--it's not particularly faster than a bike, it's not as convenient as just walking somewhere for a short trip, and it seems like people with mobility issues would be better served by a traditional scooter or wheelchair. It doesn't protect you from rain or give you extra cargo space.