In the Request Metrics section there are Request Creation Metrics under Change Mangement section of the DW package.  One of the tutorials on this youtube channel shows how to use:

http://www.youtube.com/user/IBMJazz/videos?query=reporting

Of course, when you say "open" you _might_ mean what defects are not yet resolved, rather than simply created, but using the Creation metrics in conjunction with Request Status you can get historic information.