I was just told that there’s an IE Developer Toolbar:
http://www.windowsmarketplace.com/details.aspx?view=info&itemid=2695980

At first glance, it provides a significant fraction of the utility of Firebug, such as showing DOM structure changes post-load. It also has a “Trace Style” feature, evidently for revealing the cascade of styles that apply to a particular element, but when I tried it all it did was open a window displaying my stylesheet.

It does not appear to support live editing of markup or styles.