It would be nice for members to be able to watch tags, and get notified when someone posts a topic with a given tag, just like they can watch forums. The interface could even be put in exactly the same place, in the "tag forum" display.

Seconded! We have received exactly this same request today.

Stackoverflow has this.. it's pretty awesome.

I would love this.

+1 to this idea. :) I was just doing a search for this very thing and found your topic.