Seattle seems surprisingly depauperate in tweed rides. I believe there has been just one, and I missed it. As others have also noted, some of us do a tweed ride nearly every day on by riding roadsters and "normal" clothing. I would do one, though, as a way to meet other bicycle enthusiasts.

The only real group theme ride I have ever participated in was a costume ride on Halloween morning. That was fun-- I dressed as the wicked witch of the East to match my rod-brake loop-frame DL-1. The only slightly annoying part is that other riders kept asking me how I was riding so far in high heel shoes (sparkly red ones, of course)-- to which I kept pointing out that I am not walking in them, so it is easy as riding in any other type shoe. I wanted to ask them how they were able to ride so far in clipless with all this starting and stopping, but I held my tongue.