The SiteMap architecture of ASP.NET 2.0 allows roles to be defined for each menu item, thus restricting their view to only users who are in that role. This requires the securityTrimming attribute to be added to the siteMapProvider, but I'd never been able to get this to work, and assumed it was a just a simple bug in the beta.

I now learn that it's not a bug, and the solution is pretty simple. Danny Chen explains it in this forum post. Simple really.