BrowserTraySwitch is great. Have you ever considered writing a similar program for switching the default email client?
In particular, it would be great if such a program could support Portable Thunderbird - as discussed here:
I realize that setting the default email client involves quite a few more registry entries than does the default browser; plus you need to know the path to the MAPI dll and a buncha other stuff.
Anyway, food for thought - as if you're probably not busy enough!