Web-based forums/boards have had comment formatting buttons (quote, italic, bold, link, etc.) for years. I have no theories as to why this feature is not present on any major blogging platforms I know of. Even weirder, it’s really hard to find a plugin to implement what would seemingly be a much-requested feature. But went searching for one tonight and eventually found LMBBox.
Not listed in the major plugin repo’s. Doesn’t claim support for anything over WP 2.0, but I’ve got it working in WP 2.3. Required some mods to comments.php in my theme (probably one reason why it’s not a common plugin), but seems to be working nicely (Safari of course insists on showing its usual elegant but un-styled form buttons; not yet tested in IE).