A good point brought up in another post you need a plan if you get a flat tire on your commute - you will probably need a mini pump, spare tube, etc and maybe a saddle bag to hold them. Alternatively some really good flat resistant tires, and a backup plan if you do get a flat (for example if you ride along a bus route and you can just put your bike on the bus if you get a flat). You can start with the backup plan and get your own stuff to deal with it later.
You are getting a lot of opinions on what bike to get, new and used, etc. Remember, this is your bike, so ultimately its up to you what bike you get, the cannonade, another brand of bike, a used bike, etc. A used bike might save some money, but you will have to do maintenance yourself or pay the shop. If you get free maintenance on a new bike, that could be a good deal if you don't want to learn to do it yourself. As for Cannondale vs Trek vs Giant vs Specialized - everyone has an opinion, but at the end of the day, ALL of these brands have a similarly priced and equipped bike that will be fine for you. It really comes down to two things 1) the bike YOU like best - fit, feel, looks, etc. and 2) a SHOP you like working with and will give you good service, is convenient, etc. I have learned that it can be very nice to get a good relationship with a local shop.