That has nothing to do with why they can't edit it. It's the center code being messed up, not how many characters are in it!!
Try... [b]- Shawn[/b] Server fundraiser going on [url=http://www.digitalpoint.com/tools/donate.html]over here[/url]. Please do not PM, IM or email me for product or tool support (they will go unread/ignored). aka, I *think* it's the same way your current signature is. If that works for you, it looks like it only does this to members in certian usergroups, and it's not just the center code, as you don't have that code.
Tracked it down (it was being caused by a fix for something else in 3.6.3). I reversed the 3.6.3 change they did and reported it: http://www.vbulletin.com/forum/bugs36.php?do=view&bugid=1198