I carry a patch kit, tools and a pump. I've never had a sudden flat, only slow leaks. One I repaired at work during lunch, and the other I waited till I got home.

Since I got the new tires on my bike, I haven't had any flats at all. 8.5 mile each way. There's also lots of bus routes, worst case is I'd have to walk my bike a few blocks to catch a bus. I also pass two cycle shops where I could get parts in an emergency (assuming, of course that I brought some cash with me!)

I'll have to worry about it more when I move, a couple miles longer trip, plus the route is a little more remote. I'll probably carry a couple tubes with me then.