I ride a surly cross check as my main commuter. Any bike can be a commuter really, but I like the eyelets for fenders and rack braze-ons that the cross check has. The 2011 cross checks even have rack braze-ons for a front rack in case you should ever decide you want to use one. The cross check is super versatile as it can be run single speed, fixed gear, with an internally geared hub, or a derailer. It might be a touch over the amount you are willing to spend though.
If you want to look into touring bikes you might want to check out a novara randonnee, as it can be a great bike at a killer price with on of REI's 20% off coupons.