I wouldn't want to see this feature implemented with a comma, because right now, the comma means OR when you use it in a tag page URL like http://snakeling.dreamwidth.org/?tag=misc:knitting,misc:culture:poetry.

My first vote would go to boolean. Even if only AND and OR were implemented, it would already be awesome.

If we have to use punctuation or something, I'd prefer to use &&, which is unlikely to be used inside a tag already (and could be tied in to a full boolean search if added later *g*).