It's a modern browser for modern systems.
However Tuesday we discovered that the IE9 platform preview wouldn't install on Windows platforms older than Vista SP2. The demo itself wasn't really usable as a browser: it lacked a full user interface. But despite its limitations, it was assumed that the lack of pre-Vista SP2 support was only applied to the current download.
That's not the case. In fact, Microsoft has now made it perfectly clear that Internet Explorer 9 will not work on Windows XP now or in the future. "Building a modern browser requires a modern operating system," Hachamovitch said during a press conference at the event.
For users of the older OS, Internet Explorer 8 might be the last new browser from Microsoft.
In related news, CNET reports that Hachamovitch also briefly talked about codecs Microsoft will support in Internet Explorer 9, however Hachamovitch would only "commit to H.264."