I’ve been thinking about tagging UI as well, and this discussion about tags, commas, spaces, etc got me thinking a bit more today. For one, I tend to agree that spaces are bad delimiters, and I agree that “what’s wrong with commas, anyway?”. More importantly, however, is how to get users to understand how to use a delimiter. As others have noted, they do understand the comma as a delimiter for email addresses, but search engines have trained us to use spaces to seperate words in free text fields.

So, here’s an idea - what about a dynamic UI that provides some visual feedback as I type. If I type without any delimiters, each word is highlighted seperately (two "tags"). A visual cue (perhaps a comma fading in to the right of the text field?) reminds me of the delimiter. If the user then adds a comma -- -- then the highlighting changes to show that the words are treated as a phrase, or single "tag". Here's a crude illustration of how I'm thinking it would look:

I’m sure someone can figure out how to do this sort of word-by-word highlighting inside an HTML form field…