I currently have a 3 mile commute to work and usually ride my motorcycle. Thankfully the dress code for me is polo shirt and jeans.
During the nice weather months of Phx, I wear my Firstgear jacket/pants over my work clothes. I usually wear my Sidi boots and keep a pair of sneakers in one of my desk drawers.
If the summer heat is real bad, I will usually pack my pants to work and change into them at work.