Because I love green and wear a lot of it (redhead w/green eyes) and I loved the idea of having a Harry Potter item that was a little on the "stealth" side -- nobody has ever looked at it and noticed that it's HP though I do get compliments on the bag -- I have a Durmstrang messenger bag:

http://www.wbshop.com/cat/Harry-Pott...enger-Bag.html

(As you can probably tell, my bags are just for wearing -- I have yet to wear one when biking.)

I notice that Warner Brothers is also selling a Superman messenger bag:

http://www.wbshop.com/cat/Superman/C...enger-Bag.html

Click on the different movies -- they also have Batman and Corpse Bride, maybe others!