if you opt for a rack and trunk, my new favorite is this one. I like that it's a giant gaping hole one can stuff to the gills. I'm not currently commuting but this bag would be my commuter now. my other bags have too many too small pockets and when commuting I need volume and this bag has it. it'll fit any rack but it's not cheap. I paid $99 at a LBS a couple years ago. now we use it for family bike trips to the beach or day trips with wifey. looks like it comes in red now too.

http://www.rei.com/product/847603/ti...ck-rack-trunk-

Timbuk2 Hunchback Rack Trunk - YouTube

Hunchback Rack Trunk