Ok, IE7 was just released (early 2007) and with it came a whole big set of changes as to how the browser supports/doesn't support the css standard.
As a rule, IE has been sketchy with standards support and therefore there have been a lot of "hacks" created to make web pages look similar in IE to what they look like with FireFox, Netscape, Opera, Safari, etc. IE7 has actually improved a lot of their standards support (annoying, as new css standards will soon be out).
For the Spring '07 class, we're going to acknowledge this, but we're not going to implement this. Why? millions of folks still use IE6...generally you have to remember/plan for one browser version back (or at least plan around 80% market share). 6 months to 1 year from now...design to IE7, for now, be happy about IE7, but design to IE6.
For more info on some IE7 changes (this might not make sense if you haven't done this kind of work):
http://blogs.msdn.com/ie/archive/2006/08/22/712830.aspx
Some highlights: