I should elaborate on what I mean by tags in Twitter, since I got a few head-scratching responses.
No, fitting tags into the 140 characters for a message won't work. And, no, tagging every tweet as you go is a horrible approach - no one will do it. What I've thought might work, though, are sticky tags.
Sticky tags would persist from update to update. From the web UI, you could drop "work" or "gnomedex" or "l:95051 beer concert somerandomband" into an additional tags field and all tweets from then on will gain those tags until you empty the tag field.
There could be shortcut commands to change or clear tags - ala "d someone" for direct messaging. This would be especially useful while out and about and mobile. Say you text "t bus commute" to Twitter when you get on the bus for home, then anything you might emit from then on, until you change the tags, gets tagged with "bus" and "commute".
Then, you and others could follow this particular thread of tweets via http://twitter.com/yourname/commute - or even with tag intersections ala delicious: http://twitter.com/yourname/commute+bus.
Going to be at SxSW for awhile? Drop "sxsw" into your sticky tags. Maybe someday everyone who doesn't want to hear it can filter out tweets with that tag rather than stop following you altogether.
Having some sticky tags can allow a bit of metadata and filtering hooks to follow you for awhile without requiring you to do or remember much. Keeping them as free-form text strings allows some cow-pathing as people invent conventions.
Think of these tags as kind of long-running meta-status.