I've tried playing around with the Folksonomise your files with Automator tip from MacDevCenter.com - I really like the idea of it.
If you follow the tip, you get a handy item in the Finder's contextual menu that lets you bring up a text field to enter tags for the file. Like you do with del.icio.us, or Flickr, or iPhoto. Then, Spotlight lets you find them easily. Or, you can create Smart Folders in the Finder for tags, and have these files be found (even if the tag isn't contained within the content of the file - a case that would normally make Spotlight overlook it).
It's close, but not perfect. Ideally, I'd need a key combo so I could just select a file, hit, say F5 or something, and enter the tags. Having to right-click the file breaks the flow, and isn't that much simpler than just calling Get Info, and toggling the "Spotlight Comments" text field to enter stuff (which is essentially what is being quasi-automated here).
Some pretty interesting potential - a folksonomy-native filesystem on your desktop. Rock on.