Testing 4.0Alpha on Joomla1.5.8 on XAMPP, I encountered the following error in the back-end while going to "Manage Comments". This occured both before entering any comments from the front end, and after entering a test comment.

List of joomlacomment Comments
Catchable fatal error: Object of class stdClass could not be converted to string in C:\Users\xxxx\administrator\components\com_comment\admin.comment.html.php on line 50

More details:
Database Collation: utf8_general_ci
PHP Version: 5.2.6
Web Server: Apache/2.2.9 (Win32) DAV/2 mod_ssl/2.2.9 OpenSSL/0.9.8h mod_autoindex_color PHP/5.2.6
Web Server to PHP interface: apache2handler
Joomla! Version: Joomla! 1.5.8 Production/Stable [ Wohnaiki ] 10-November-2008 23:00 GMT

Great project. I'm looking forward to your of legacy-free 4.0Stable!