There seems to be a bit of confusion about Go Bags and Bug Out Bags. Judging by this article http://speedbird.wordpress.com/2008/02/22/against-go-bag-silliness/ it's clear that people tend to think that they are one in the same. The difference, however, is that a Go Bag is used for everyday purposes and a Bug Out Bag is used for survival purposes. You should have both types of bags at the ready at all times.
A Go Bag is used every day and contains all of the items you need to survive the day (lunch, water, cell phone, iPod, pen, paper, work, keys, etc).
A Bug Out Bag is filled with items that can help you survive should you need to evacuate at a moment's notice (clothing, shelter, food, and other emergency supplies).
You should have a Bug Out Bag at your home, in the car, and at your office as you never know when disaster will strike. Having a single Bug Out Bag is fine, however the thought of carrying such a comprehensive bag with you at all times may be a bit unnerving (and in most cases unnecessary).
A smaller, less comprehensive, more useful on a day-to-day basis Go Bag should be carried with you at all time.