Some of us have been using our bike for transport for years, some people have never thought of it. But with the way gas prices are, a lot more people are giving it some serious consideration. I mean, it's gotten so bad,
George W. Bush of all people is suggesting people drive less!!!
It's unfortunately not so easy to just hop on your bike and take off. There are things to be prepared for: carrying stuff, dealing with the weather, and fixing minor problems. To help those of you that are giving the idea some thought, here are my suggestions to help your choices out:
Rain gear is a must no matter where you live. Rain happens.. as does freezing rain. And it will act as a windproof shell in the coldest of temperatures. In my experience, Burley makes the best. Whatever you do, get something with big pit zips to help regulate temperature. It can get a bit hot in there.
Cover your hands and feet. I haven't found the holy grail of gloves, but there must be something out there that is waterproof/windproof, not bulky, and durable. I had previously had luck with a Campy glove but I discovered it lacks the latter attribute. It's hard to avoid bulk if you're in really cold weather. For the feet, get some wool socks (I prefer Wigwam Ultimax synthetics which work just as well as real wool). If you do cycling shoes, try Gator's neoprene booties, otherwise get some waterproof hikers or running shoes (like Montrail's awesome trail runners) and consider adding some gaiters.
For the coldest weather, think in layers. I suggest breathable thermal underwear with the rain gear over. Get a thin fleece or neoprene headband that will fit under your helmet, too.
Lukcily, outside of these pieces, high end clothing is not necessary. I'm content to ride in shorts and a t-shirt and use arm warmers, leg warmers, and a windproof vest to modify the basic outfit to work in most every change in temperature without the bulk of spare clothes. I prefer t-shirts that have to do with cycling :) and Dickies 13" shorts because they're super tough, long enough to cover the knees when it's slightly chilly, and because they have a stain release finish that's slightly water repellent.
I strongly suggest against leather. It's HORRIBLE when it's wet. There exist synthetic leather alternatives for every single leather item. Though, if you're hooked on the Brooks saddle there, there is obviously no alternative, but that's another story..
Get fenders! When it has just rained but it's too hot to put on the rain gear, you will avoid the wet feet and the striped back that is commonly associated with the Fool-Who-Doesn't-Use-Fenders. Also, they will keep your bike cleaner and reduce maintainence.
Get some tools. Pedros and Park make a wide variety to use at home. It might be good to have a few things laying around if you know how to do some wrenching (if you don't, LEARN! It's very easy stuff and will save you some good money). But definately have some tools with you. A Crank Brothers Multi-19 multi tool will handle just about everything but your tires. For that, get a AC International Quik Stik tire lever (you only need one), a Park glueless patch kit and a tire boot just in case. And don't forget you'll need air. I like my Planet Bike Roadie.
You need to see and be seen: get some lights. Marwi's BULLITe is high powered, rechargable and self-enclosed. It's got a nice, easy to adjust clamp and comes off easily so you can take it in the store with you so no one runs off with it. For the rear, Cateye's SL-LD100 is just as easy to remove, fits on anything, and has a cool waterproof magnetic switch.
And now for THE MOST IMPORTANT THING OF ALL:
Invest as much as you can in a good lock. That bike is your transportation-- you depend on it. So take care of it! I personally like Kryptonite's New York Lock series, such as this ulock that features a real key-- with a convenient light on it.
That should get you on the road. Be aware that a bike that gets ridden a lot in all sorts of weather will not be as maintainence free as it was just going on sunny recreational rides. So it would make sense to learn how to work on the bike. It would also make sense to get a mechanically simpler bike, like a fixed gear, but that's another story. :)