The only thing going in the back-pack is my laptop (5.6 lbs iirc). I'm not too keen on the idea of it riding mounted to a rigid rack, especially on the dirt portion of the ride. But I'm a newb, and haven't even tried the commute with the back pack yet.

I'm trying to hold off big purchases, but will be ordering the following tonight:
-Multi-tool - Ether the Topeak The Mini Plus 18-Function or the Crank Brothers Multi Bicycle Tool (17-Function)
-Pedro's tire lever
-Portland Design Works Shiny Object CO2 Inflator with 16G Cartridge
-innertube
-spare link

and possibly this
Amazon.com : AGPtekŪ Roswheel 2013 New Waterproof Bicycle Cycling Frame Pannier Front Tube Bag w/ Headphone Jack (Including Clear PVC Window Pouch for 5.5" Cell Phone, Reflective Strips for Safe Night Riding) : Bike Handlebar Bags : Sports & Outdoors
to hold all that stuff above and to replace this