Sure, that's the ideal and gradually becoming more common. (Heck, the Two Worlds development in Mountain View -- ground-level retail with residences above -- dates to the 1980s.)
But again, the trick is figuring out a way to force people to only take jobs within their building and/or move to the building where their job is (and move both when either changes). It just doesn't work that way in the Real World.
I stated this but if you ask most people whether they'd want their office to be moved close to their house, I imagine most people would want their office to be close by. And people are willing to pay a premium for this.
Even if they don't take jobs in their building, at the very least this will cause other people in nearby residential zones to be closer to other jobs. Of course people will still use cars (and traffic will probably suck! But that's what you get for driving). But you'll reduce how much people need to drive.
Denver is also investing quite heavily into public transit, specifically they've been working on a fairly impressive light-rail system. [1]
At the end of the day, it's simple density that allows me to live inside the city and bike 15 minutes to work. City life isn't perfect for everyone but it does offer some great benefits if we go dense.
[0] http://www.rtd-denver.com/documents/financialreports/strateg...
Put housing above supermarkets! Offices inside apartment complexes! Make it easy for people to walk around.
I think that people don't need convincing not to use their cars... it's more that the space needs to be built in a way to where cars are not necessary.