I took a quick tour of the social bookmarks tools mentioned in Clay Shirky's piece on social link management. Mainly, I was looking for two things, RSS out in some fashion, and an API in.
Surprisingly, most of the services had at least an RSS feed for an individuals bookmarks, and often a number of other different outbound feeds, such as most recently posted links. APIs were harder to find. Spurl at least tells you they have one although they have to vet you before they'll let you in on it. The rest were remarkably silent.
Then with a flash of insight, I realized they all have some form of bookmarklet for posting to the service, and often for accessing other additional features like all the goofily named gobbledygook that Dude, Check This Out! gives you. With a little reverse engineering, therein lies some semblance of an API. Given that it's written in JavaScript, it ain't gonna be SOAP or XML-RPC.
One other thing I also learned, services like Furl and Spurl (are these two twins or what?) add archiving of bookmarked pages along with good old bookmarking. This is something del.icio.us doesn't support. I can somewhat see the utility, although for me, it's not often the case that a 404ed link is all that useful, even if its contents are stashed. Different strokes for different folks though.