I published a step by step tutorial on using Master Pages and Site Navigation last week. One of the questions that someone posted in the comments was whether there was an ability with Site Navigation to add custom properties or settings on a per-node basis. For example: a custom icon property (eg: imageUrl) for each item in the Site Map that could then be later rendered by the new asp.net 2.0 menu control on a page:
<siteMapNode url="default.aspx" title="Home" description="The home page" imageUrl="home.gif">
Betrand just published a cool blog post + source code sample that shows exactly how to-do this. You can read it here.
Hope this helps,