A lot of people have been writing about this recently, and I can't take credit for discovering it. I first saw it here and then again here, but I'll recap it briefly on this blog for anyone who is too lazy to follow those links.
Under normal circumstances, IE won't allow you style tags that it doesn't recognize. But there is an interesting quirk in IE that will allow you to work around this and style unknown elements. All you have to do is create an element whose tag name is that which you desire to style, like so:
border: 5px solid #f00;
Now IE will happily style all <myNewTag> elements with a red border, a blue background and bold text.
This goes for any tag that IE would not normally recognize, but it may prove especially useful in handling HTML5 down the road.
So there ya go... another IE quirk "to the rescue."