The patch just went live on Windows Update. If you run Windows or Microsoft Updates manually via the browser or Vista Update program, look for references to any one of the following (depending on your OS):
"Security Update for Internet Explorer 7" (or 8, 6, etc.)
If you need to download and install the update manually (or have a lot of machines to update, or have older versions of IE), try this search query on Microsoft's site for MS08-078:
If you are otherwise current on updates, and use Auto-Updates, you will get this patch sometime during the next few days. Personally I would do a forced check to be sure.