Basically, as far as I know, you can't autohide it.
You can set it so that it is not always on top of other windows (The Windows-Spacebar key combination will bring it to the foreground if it is hidden behind other windows). But, again, as far as I know, there is no auto-hide feature built into the sidebar in Vista.
Desktop Sidebar is a different story as I believe they have an auto-hide feature..