Microsoft released platform preview version of IE9 today and can be downloaded from here. According to Microsoft, Platform preview version is not a full browser, its intended mainly for web developers to give a sneak preview on upcoming web platform technologies coming in the next version of the browser. For more FAQ’s please visit the IE9 site here.
It also supports side-by-side installation along with IE8, but nothing is mentioned about the side-by-side installation with IE9 beta. So its up to the users to decide on the installation….