Dave Winer is impressed with the Flickr contacts RSS feed</a">, but I think he'd be even more impressed with the way the del.icio.us inbox works. You can subscribe to people's bookmarks, bookmarks tagged with certain things, bookmarks from people tagged with certain things (I subscribe only to Jon Udell's bookmarks tagged with 'screencast', but he hasn't added anything since October). I don't think this feature exists, so if it doesn't, consider it a request: you should be able to ignore users or tags as well. See my del.icio.us inbox for how this looks; there's an RSS feed of the whole thing at the bottom.
Since Flickr sort of offers an inbox with the RSS feed for contacts, it shouldn't be too hard—since the same company that owns the photo sharing service now owns del.icio.us—to create a Flickr inbox. Basically one RSS feed, meaning I subscribe once, but with options of which tags of photos and which users' tags of photos to include, and an ignore functionality, with a web interface to choose what to include in the bucket and what to exclude.
Also I'd love to be able to have everything in someone's inbox also come into my inbox. (The friends of my friends are also my friends.) And not for individual services, but the entire Web.