When I was meeting with Canon recently discussing their new equipment, they mentioned something that I'm not sure I understand, but it may explain the difference in the shutter feel/sound between the 20d/30d and then new 40d.
He said the opening curtain is now "electronic" instead of mechanical, the closing curtain is still mechanical.
I didn't ask him to explain this in detail, and I have no clue what he meant by that. He did mention the reason is live view, because the "opening curtain" may need to open and close many times when a picture is not being taken, the closing curtain only operates when you take a picture.
By electronic all I can think of is they are using some type of magnetic/motor system rather than a "spring" type system to open and close the first curtain.