The most interesting part of this thread is how often I find myself disagreeing with my fellow Americans
I would use taxi and cab interchangeably (the full name of that particular vehicle is actually "taxi cab"), whether I hailed it on the street or called for it. A car service is a pre-scheduled, fixed rate kind of thing, like the limousine service described above - we'd use the term limo service more often than car service here in Chicago.
We definitely refer to our natural gas used for heating and cooking as gas. Maybe the Yahoo Answers person lived somewhere with predominantly electric heating and cooking? Or maybe he was that special brand of stupid you only get on Yahoo Answers
I call any sort of shoe designed for sports a gym shoe, or perhaps a sneaker. I seem to be unusual in that, though. I think that's both generational and regional. I would use tennis shoe specifically for those canvas shoes. These are pumps: http://www.6pm.com/softspots-sherise-black-velvet-sheep-nappa
I still hear bum as in hobo regularly. I think there's a general trend more towards just saying homeless guy/lady, though. I do hear and use "can I bum a *whatever*" here.
Thong was used for flip flops much more often when I was a kid. I'd know what someone meant referring to thong sandals, but my first thought would be underwear.
Pants are trousers, but a man might refer to his shorts and mean his boxers. You'd have to pay attention to the context.
We definitely don't use fortnight. I only really encounter it in books and foreign media.
Durex is a brand of condom, by the way. I wouldn't say "I need a Durex" in the way I'd say "I need a Kleenex," but maybe English people do. Rubbers are a synonym for galoshes - rubber boots that keep the rain out.
Bush isn't just the hair down south
. It's also a particular type of plant. I'm pretty sure that usage is international, though, and dawnfire was just being goofy.
If I say "last summer" instead of "last July," it's often because I don't really remember what month it was
I use mooning strictly in the sense of illicit butt presentation. I would recognize "mooning after" someone if I saw it, but I don't know anyone who uses it. If I was feeling old fashioned, I'd use "pining after" or "pining away for."