Just found a nice post about the Mailplane's auto-bcc feature on the 37signals product blog.
If you’re interested in site specific browsers, listen to this podcast. Mailplane has been mentioned as one of the prime examples on how a site specific browser should work.
I very much share their vision and passion. Web applications such as Gmail are getting more important by the day. And site specific browsers are here to implement the "last mile," to free the web application from the browser tab and to combine the best of web and desktop world.
It's a great listen, much recommended.