Go for the bike commute! You'll never regret it :-) Well, you will maybe, on the days it's pouring, but every bike commuter has to do alternative transport once in a while. The sense of freedom is very hard to give up for a car, once you're used to it.
Personally I'd ride in athletic wear and change. It's more comfortable and saves wear on your nice clothing. I'd probably also take a washcloth and wet it well before wiping down in a bathroom stall. If you take it easy and don't over-dress you needn't get too sweaty, depending on the weather of course. If you want to look a little less bikey you could wear a skort, and take off helmet, sunglasses and any hi-viz gear after locking up. I bet hardly anyone will notice, if anything they'll be impressed, and a little curious :-)
Another good reason to not get too sweaty is that you'll probably want to bike home in the same clothes. A good tip I read here for drying clothes without hanging them up is rolling them inside a large towel, and then putting it into a paper bag.
Do try out your route first, and leave plenty of time for your first commute. I can almost guarantee that you'll enjoy it! You'll need to practice changing a flat at some point, but the chance of that happening on the first few commutes is rather small if your tires are in good shape.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett