As far as I have heard, all of the recent Canon EOS cameras come with the software to do what you want.
There is also a company that specializes in this type of software for Canon cameras.
I just sold my D80 and eagerly awaiting to switch back to canon. I was originally going to get the 40D but the XSI just welded its ugly head. The main thing i want is live preview (here are some great demonstration videos: http://www.infoborder.com/Digital_Cameras/Video/CanonEOS450D.php ).
I also do a lot of studio stuff where i can hook up the camera to comp or laptop and it would be amazing if i could frame things on my 17" laptop screen or even my tablet. I know the 40D has this feature, anyone know if the XSI will? The XSI has pretty damn close to everything i wanted to upgrade to from the D80. The main thing is the FPS is still stuck at 3fps, but thats about the same as the D80, so i don't really lose anything.