that looks super easy to duplicate. in fact, i think i may, i'd like a new diaper bag
it looks to me as though the straps are separate pieces though why you wouldn't make it one long tube as K.E. suggested is beyond me, think of all the layers to stitch through!
i'd add the side snaps as well, the bag will gape open if not. and some stiff plastic or cardboard (cover with lining fabric and make it removable for bag washing, replacing etc) would help out the bottom.
have you got a shot of the inside? i assume it's pretty basic, 1-3 side pockets on one side and another zip pocket on the other seems to be standard. the change purse is cute too.
if i ever get around to making one up i'll be sure to post some pics (i'm still unpacking around my new babe and the 6 y/0).