Lands End has a great diaper back-pack in black (well, they have other colors too, I just like black because then either me or my husband can wear it without feeling "weird"). My only complaint is that its a little large when fully packed. But I do pack it really full because I have a 3 1/2 YO and a 5 month old.
It has a pull-out changing pad in the back in a flat pocket (where I also stash a coloring book and crayons), 3 pockets of graduating sizes and a bottom pocket. I use the smallest pocket for my stuff -- wallet, cell phone, address book. The next size is for snacks for my older child and an insulated bottle bag for my baby (I always carry breast milk because we hate nursing in public), and a bottle of water for me. The largest pocket holds a first aid kit (my kid is always hurting himself somehow), diapering stuff, cloths for the baby, cloths for the older kid (lots of potty acidents), a small blanket, spit-up cloth and a couple of toys. The bottem pocket I use for icky cloths (in a plastic bag). So, its huge but it holds everything i need for a full day out with both kids!