Microsoft enabled "SmartScreen application reputation service" to improve download protection for IE9 beta users. This feature works together with SmartScreen anti-malware service that protects IE8 and IE9 beta. You can experience the protection of SmartScreen app yourself by ensuring SmartScreen is enabled. Just click the Tools Button | Safety | Turn on SmartScreen Filter menu item, then choose Turn on SmartScreen Filter in following dialog.
How does this work? "When you download a program in IE9 a file identifier and publisher of the app (if digitally signed) are sent to a new app reputation service in the cloud. If the program has an established reputation there's no warning. If the file is downloaded from a reported malicious site, IE9 blocks the download, just like IE8 does. However, if the file doesn't have an established reputation, IE lets you know in the notification bar and download manager, enabling you to make an informed trust decision," explained Ryan Colvin.
Microsoft has set up two files for you to try it yourself: "First, download IE9 and make sure that SmartScreen Application Reputation is enabled. Next, download an application with established reputation: freevideo.exe, or with unknown reputation: freevideo.exe."