I am working on my implementation of HTML5 WebWorker shim via Google Gears plugin (currently). Part of what is interesting is that the WebWorker has a lot of the other HTML5 APIs available internally, so I am also looking to bring in other shims / write my own if I want the project to be complete and provide an actual fully-functioning Worker-alike and SharedWorker-alike.
As a result, I've decided to start a project to collect the available shims. Rather than just the Web Worker, I will try to bring together as much of the HTML5 js apis as possible. The project is called "html5-shims" and is hosted on Google Code.
I have added a page to the wiki for my worker shim/html5 shims project which lists other shims available, the relevant specs, etc.
HTML5 Shims : Links & Resources