There was a small snafu when Opera changed back to their old rss icon in weekly builds stating that mozilla was requiring the signing of an agreement. I’m uncertain if that has been sorted out but there have been some posts regarding usage guidelines of the icons:

From a quick read it appears their major concern is the use of the icon for proprietary feed formats, those that can’t be implemented by FOSS developers. I can see that concern coming about when they worked with MS/IE7 and is probably a valid concern. There are also guidelines about color, min-size, etc. It will be interesting to see what the final guidelines look like and what they require.

You can get HQ versions of the icon at though not all versions may meet the guidelines it appears.