Go back to Page 3 of this thread where JohnnyBackpack had two posts - he's the commute runner - to and from work everyday.
Since I read those posts I could not get it out of my mind (especially when stuck in traffic!). I'm a prime candidate for it as well - 6 miles from work, already running 80-100 miles/week, shower available at work. Also a bonus - a track available right by my work.
So, I went out and got this backpack - $40
http://www.rei.com/product/762540
It's frikken awesome!!! I sweat ALOT, and this backpack has a good wicking mesh on the back and also a whole "hyration compartment" (where you can put a hydration pack) that's inbetween the main compartment and my sweaty back. It's honestly very comfortable and doesn't impede any form - haven't done a tempo or fartlek with it, but I don't think it'll be a problem. It just enough room for slacks, undershirt, button up shirt, socks, undies, wallet, keys. I'm thinking of driving in Mondays and bringing in my button up shirts for the week (or dropping them off on weekends). For lunch, I can keep some crackers, fruit, etc. at my desk and put a sandwich in the backpack. I'm starting off by commute running 2-3 times/week, but might mix it up w/ the bike too as training and/or weekly mileage and race schedules change.
If you have the luxuries of the following:
-Office is close enough for you to run
-Shower available at office
-Able to keep shoes/belt at office
You should really consider it - again, read the JohnnyBackpack posts on page 3 - good luck!!!