WebMonkey has a good article explaining why hash-bang URLS are a bad idea.  If you’re not familiar with them, they have  #! symbols at the beginning of the path, e.g. http://twitter.com/#!/username. They rely entirely on JavaScript to parse and therefore make the site inaccessible to browsers without  JavaScript (or those with it turned off by the user), assistive technologies that people with disabilities use, and search engines that haven’t been modified to recognize them.