If you'd like some more information on bike commuting, I can email you a PDF of a Bike Commute Manual type thing that the League of American Bicyclists put together. It covers the entire range of stuff you're concerned about and is pretty comprehensive.

Another tip: Does anybody else bike to your work? If so, how about teaming up and having an experienced commuter escort you until you feel comfortable? If you're worried about slowing them down, remember that all experienced bike commuters love to help new commuters -- as evidenced by this forum -- and they'll gladly go your speed to help get you on the road.

I like to use the bus as my flat tire backup. It's sure nice to know I can just toss my bike on the bus and get to work in a reasonable time.

As for riding in the rain, I second Grog's comment: Nobody's going to melt out there. Riding in the rain, especially in the summer, can be glorious -- you get a cool, refreshing rinse-off and arrive wet but not sweaty. Just be sure to have bright blinking lights so drivers can see you through the rain. My rule of thumb: If drivers have their windshield wipers on, I turn on my lights. (OK, actually I ride with lights on all the time, but that's a different story.)

Most of all, bite the bullet and do it. When you decide "I'm going to ride," don't even let the thought "I could drive..." enter your head. That instant of wavering will kill you every time. You can do it! We're all here rooting for you, eagerly waiting to hear how your first ride goes. And your second, third, fourth, etc.