Lucky enough to be able to get away with jeans and a polo shirt at work which is a 6 mile commute, although I do change from a t-shirt to the polo when I reach work. What has saved all my jeans/pants is the plastic chain guard that came with the bike, and which I believe should be readily and cheaply available at a LBS near you.

Too new at cycle-commuting to know what I'll be doing when it's raining though.