I wear shorts and a t-shirt on the commute, then change into work clothes in the copy room. I just keep the work clothes neatly folded in a really big zip-top bag in my backpack. If anyone ever points out that my shirt is a little wrinkled, I explain that I velo-commute (I find using the word "velo" adds some "gravitas"), they are so enamoured with my commute option they forget all about my wrinkles.

Changing at work also means I don't have to worry about adding fenders to my bike. If I get striped by a puddle, it's ok, since I am changing anyway.