Canon just announce a special 20D for Astrophotography - Canon 20Da. But it will only be available in Japan.
Anyway, according to their website, which is in japenese language, but got some chinese character... I think the main difference is:
1) it is 2.5 times more sensitive in H-Alpha than a normal digital camera... meaning the internal IR-cut filter is remove.
2) it can focus through the LCD and has a 10 times and 5 times magnification mode(I think so) for accurate focusing.
The japanese website is:
http://cweb.canon.jp/camera/eosd/20da/index.html
Those who understand japanese may want to read the website and see if I give the correct information or not...

So, any of our expert astrophotographer getting one???

Have a nice day.