Looks OK if limited to posts, optional for each post, and implemented as a site feature, not as HTML snippet pasted manually, for the reasons Wilka gave. But in that case it should just be part of standard profile information. And if it's part of standard profile information, maybe it shouldn't show by default on posts after all.
Kaj_Sotala suggested here that some people may wish to add signatures to posts (i.e. top-level posts, not comments (hell no!)) to link to the author's homepage and such, and this idea was supported (poll). I suggested that we make such signatures into an official looking standard template, and this suggestion was upvoted. This post contains my design for such a template. The last time I learnt HTML was back in 2003 when I was about eleven, so this is probably bad code by modern standards, but I'm hoping that people will criticise until we have a version that looks good on all browsers.
Code (Improved by Dreaded_Anomaly):
Which produces a signature like the one at the bottom of this post. To use the code in the article editor press the HTML button and enter it at the bottom of the page. (Note that having your image be 64*64 to begin with will mean that it doesn't need to be scaled. Scaling sometimes makes images look weird or pixelly.)
I suggest that everyone who uses such a signature writes about themselves in formally and in the third person. Think of an "About the Author" section on the dust-cover of a book. This will raise the status of the site by making it reminiscent of an edited publication.