I've been using my Buzz for a few weeks and I have some suggestions. I use this mainly for urban commuting via mass transit.
First, as others have said, make the bag just a bit bigger. More precisely, a bit deeper. It doesn't have to be much but any bit would allow for more storage. It's pretty tight right now. I realize you don't want the bag to look weird. But I think you can create a bit more space without sacrificing good looks and fit.
Second, make the inside linings a bright color (like a dayglow green or orange). This will make it much easier to find your stuff. The dark slate/black you use forces me to have to hunt for my stuff. Don't make it white as that will show dirt too easily.
Next, make a zippered little pocket on the outside back (the pocket itself is on the inside of the bag so that you only see the zipper, not the pocket, from the outside). I like to store a few things I need everyday like transit cards, etc. But I want very quick and easy access. I don't want to have to open either of the two larger pockets.
On the iPod/cellphone outer case, the flap should have a hole in it to thread your earphones through. I find that when I place my iPod in the case and close the top flap the cord to my earphones bends and over time I don't want this to get broken.