Marjolein Hoekstra and James Corbett hacked together something quite interesting!
I’ve not dived into it to deeply but this is how it basically works: You can add a link-tag to the header of your page with the url of an opml-file (<link rel=”alternate” type=”text/xml+opml” text=”Opmlmanager userlist” xhref=”http://www.opmlmanager.com/userlist/userlist.opml” mce_href=”http://www.opmlmanager.com/userlist/userlist.opml” / >). A visitor which uses a tool for autodiscovering, which for now is a bookmarked page that wil make a little div popup at the page your watching that will you show a link that will take you to a grazr-page that will display the opml, will directly detect the opml-file. Just like autodetecting rss.
Now all we need is a add-on for firefox, so you don’t have to go the bookmarket to discover opml-feeds, but that will show an opml-icon (is there a final design yet?) on every page containing an opml-link in the header. And of course you should be able to configure what should happen once you click that opml-icon.
What would one do without the Dutch and the Irish 
[...] In case you’ve missed it, the opml auto-discovery bookmarklet i posted about a few days ago, has an improved succesor: a OPML Auto-discovery Extension for Firefox (download). [...]