In a recent article on the IE Blog, Justin Rodgers talks about further CSS improvements in the ever more impatiently awaited IE 7 beta 2. His message is that CSS hacks will start to break in IE 7, and I fully agree.
Nearly two years ago I warned against the excessive use of CSS hacks, because I envisioned a situation like this. Web developers who rely on CSS hacks are going to have serious problems.
Take my advice and stop using CSS hacks right now, except for the very few safe ones.
CSS Hacks (at least how I define them) are when you use tags to get certain browsers to ignore or use styles differenly than other broswers. A classic one of old is to use an import media rule to tell IE to do one thing, and Netscape another. Looks like these old tricky ways will be problematic in the new version of IE.