Like at least several other monks (that I know of), I have some "template" information in my signature. With that patch, I'd end up typing my entire node content within the "sig" unless I remember (every single time I post) to move things around.
But I think that problem can be addressed by a minor improvement to the patch: Only add the tags if they aren't already present in the signature. Then I can update my signature one time to incorporate the new tags (which I can cut-n-paste from a quick "preview") and never have to move parts after that.
Other than that, I worry that perhaps we need to think a little more about the tag. I'm anything but a CSS expert, so I'll ask some dumb questions. I guess div is a better choice than span since a signature really shouldn't be an in-line entity. Someone already gave an example of apply custom CSS to all sigs. Is that type of wildcarding common in browsers supporting CSS? Or should we have both a per-user tag and a all-sigs tag? Perhaps something like <div class="pmsig" id="pmsig-22609">?
I'd rather get that right the first time than have to worry about backward compat when we realize we didn't as good as we could.
Thanks for the idea, adrianh, for the patch, chromatic, and for the future feedback, y'all.