So, I build a dock into my bag and wired it through to an inner pocket where theres a portable battery. Now, whenever i slip my iPhone into that pocket, it gets charged!!
Step 1: Buy all the crap you need!
then, the awesome part is the tekkeon (i have no commercial relationship to them, just a happy customer!) rechargeable mobile power unit, purchased for $36.95 on newegg.. basically, its battery pack with a 4,000 mAh capacity (iPhone's battery is 800 mAh i believe) that outputs power to a usb port! perfect for the iphone!
obviously, you need the sync cable, and the iphone itself.
oh, you'll also need some krazy glue, or other adhesive that works on plastic.. might wanna get a sharp knife too..