MOST of the corporate world is still using IE6. Large organizations that I have dealt with in the past are still very much behind the times when it comes to new technology. One major firm that I worked for is still using Windows XP as the primary operating system. Granted they are rolling out Vista, but it's a very slow process in large organizations - especially when some custom programming will not work on the Vista Platform.

As for using a variable size on pages, when I used to design years ago I did this. It's commonly referred to as a Liquid page, or Fluid Design. We had multiple splash pages at various sizes in order to capture the necessary market.

I think operating in a liquid design these days is a suicide mission given that people have 24" monitors and others still on 15". If you are concerned about how the page looks, and you want it to look the same for everyone, you specify the width of a page. Not doing this can leave a page looking very "empty" as the text stretches accross the screen.