If I configure Apache to serve /favicon.ico as MIME type image/vnd.microsoft.icon, instead of image/x-icon, will it break on any browsers? Wikipedia's favicon article indicates that image/vnd.microsoft.icon is the "correct" mime type, but that doesn't help if it means some users won't see it.
I need to support IE6+, as well as modern browsers (FF/Chrome/Opera/Safari).
Google serve /favicon.ico as image/x-icon, and I'd guess they know what they're talking about (or at least, what's compatible).
If you are able to set up a website serving the favicon you could use something like http://browsershots.org to test compatibility with a wide range of browsers. It takes a screenshot of the whole browser window, including the displayed favicon.
Off the top of my head, image/vnd.microsoft.icon will work only if the image actually is a real ICO file, while image/x-icon is less fussy, and will also work for bitmaps and GIF images. (Non-ICO files with the .ico file extension are, I believe, quite common.)