I'm not so much interested in which method to use for publishing (like this question), I'm more interested in how I should go about marking images as "selected for publishing" and keeping track of images that have already been published (smart collection?). I've not published any yet, so I don't know what Lightroom does for me and what I should be doing for myself. I was thinking about having some "meta" keywords that aren't exported as meta-data but are indicate whether an image is ready for flickr and when an image has been published? How do other folks do this? I'd prefer to know how it works before pushing the button for the first time :-)
When you configure the Flickr publish service (either Jeffrey's or the built-in one), you create one or more "publish collections" that correspond to your Flickr photostream or sets on Flickr. When you add a photo to a publish collection, it appears there under a "New photos to be published" heading. After you upload it to Flickr using the "Publish" button, it stays in the publish collection under "Published Photos". If you ever modify it, it again stays in the publish collection, but under another heading, "Modified photos to re-publish", and again, you can click Publish to upload the modified version, and it goes back to "Published Photos" in Lightroom.
So Flickr keeps track of your main bookkeeping concerns: What do you intend to publish to Flickr, and what has been published to Flickr. That's really the joy of publish services in Lightroom.
Also: An entry in the edit history is created for your images whenever they're exported or published, so you can always go back to the published version in Lightroom. Jeffrey's plugin has the ability to also record a "Snapshot" of each image when it's published; this appears under "Snapshots" on the left side of the Develop module, and makes it even easier to go back to the published version(s) after you do some editing.