First off, kudos! I've been tinkering about Shadowbox a bit and have found it to be an excellent media viewer. The code is very clean and the comments in the source are invaluable. Thanks!
I do have one request. I've hacked about a solution for one particular project I'm working on, but would like to see this added in future versions.
I'm currently whittling down exactly where I've got it worked out, but I had to mess about with both the CSS and the JS to keep only the fades in the animation and the resizing to a minimum. Better would be if I could set up an option to only fade and not resize. Or if one specifies the width and height of an image in the rel attribute, Shadowbox would calculate the final height and simply fade in the box rather than animate the resize.
The only other workaround I could do at the moment would be to have all images be the same dimensions and set the initialHeight and initialWidth to those dimensions.
I imagine I'll be playing with this some more to see if I can come up with a solution myself, but I think others could use this feature as well.