I have given up on staying dry in the rain. Anything waterproof enough to keep rain out usually also keeps sweat in so if I'm not wet with rain I'm soaked with sweat.

What I do is dress down for the commute with shorts (actually a baggy style swim trunks that dry quickly) safety lime t-shirt and sandals (I'm thinking about getting Crocs this year) with no socks. I bring my change of clothes along with a towel bundled up in a water proof bag.

I find bowing to the inevitable improves my attitude towards riding in the rain and it is often more enjoyable that way.