I've added a TagLine variable to the header template. (The tagline can be edited in the Theme tab). Taglines underneath headers are increasingly common in blogs, and this allows one to set up the tagline without having to change the header template (which can get replaced by themes).
I still have to add support for the tagline to the existing themes and the custom theme creation script, but even as it is this should make things a bit easier for people who want to use taglines.