Many systems use tags these days but I think they differ in a single but important way: is the tag name allowed to include a space and if not, how are multiple words combined? One way would be to use underline characters or no characters at all. I do dislike both of these methods as underlines are ugly and it might be a difference if two words are used separately or combined. So considering the space as a part of the tag would be the way to go I guess.
However, if I enter tags to a URL posted to del.icio.us I have to separate tags using spaces. On the web however, separating keywords with a comma is much more common.