Toyota, I was thinking you needed that truck to carry all that stuff in! LOL! If it all fits in a backpack, that must be one big heavy backpack.
I answered this question a long time ago before I actually got my MBP. Now I have had it for a little over a month so I know how I use it, and it really varies based on where I am going. This is one of the things that is so great about mobile computing, it really is flexible!
If I'm traveling, I take everything - 13" MBP in an Incase Deluxe Protective Sleeve, MagSafe charger, Toshiba 500GB portable USB hard drive, iPhone 4, iPhone USB cord, headphones, Nook Color, and USB cable for my Nook. All of this is just carried in a basic Nike backpack.
Other times, depending on where I am going and what I am doing, I always have the computer in its sleeve with my headphones in the outside pocket if I'm not going to use it enough to need a charger. If I think I'll need to charge it then I'll throw the sleeve in a canvas messenger bag along with the charger. This computer is so light and portable, I love it!