OPML as the standard format for directories, I like this.
It could also be extended, directories are not only collections of sites, but can also represent complex sites.
Take this e-commerce site, for example. It's in Italian, sorry, but anyway it's a standard e-commerce site, where you can navigate trough a... directory to find what are you looking for, then add it to your shopping basket and buy it.
Now, guess how the editor manages this directory.
Yup, with Radio UserLand! Here's how this site looks like to the editor in Radio:
At this point, adding a link to the xml directory into the html code of the home page would be very easy.
In the global directory, this site would look like a sub-directory, managed by the editor of the site.