http://del.icio.us/inbox/user shows a user's subscriptions. For example, here's mine. It doesn't come in a nice structured format, so you might need to do a little screen scraping, but one could start building out an interesting network on top of this.
Note that in del.icio.us you can subscribe to both tags and users.
So of course, by beating the hell out of the service with a crawler, you could try and construct a global picture of del.icio.us. But more interesting to me would be how much social navigation is/could be generated using purely local connections. Other than the front page, there's not much of a shared global picture, by intentional design I suspect.
And just to add one meta level, you could monitor inboxes and generate RSS, so folks could subscribe to someone else's inbox and subscriptions. Basically, with a little effort, you can monitor a person's inflow and outflow.
Of course, Joshua Schachter has all the raw data at his fingertips too.