You could right-click on it and choose properties to get more options but it seems the closest is 'auto-hide' which only partly solves the issue.
I'll just add to this because comments rightly point out that Vista let you do this.
From The Windows 7 Team Blog
A central Windows 7 tenet is that the "User Is in Control"; that is, we empower users to take ownership of their desktop looks and functionality. From little things, like allowing users to arrange their Taskbar icons as they see fit, to enabling users to control the number of icons on the Taskbar. Windows 7 “removed” the System Tray Icon area. By default, almost all the tray icons are concealed. Consequently, it is safe to assume that large number of the notification balloons will also not be visible and most users will not see them. You can read more about the updates to the Notification Area here. To compensate for this lack of notification, Windows 7 Taskbar offers Overlay Icons and Progress Bars. By using overlay icons and progress bars, your application can provide contextual status information to the user in spite of the lack of a System Tray Icon area and even if the application’s window does not display. The user doesn’t even have to look at the thumbnail or the live preview of your app – the Taskbar button itself can reveal whether you have any interesting status updates. This functionality is part of our commitment to provide users with easily accessible information about an application's status without any extra clicking.
If you read more on MSDN about getting applications ready for Windows 7, there is a fair bit on how to use the taskbar buttons to relay information to the user, something which used to be done via the system tray. If they hide the notifications from the tray and allow people to hide the taskbar, you aren't going to see any important messages.
So I doubt you will find any way to do this at all, I'm afraid.