Attached is an course backup of a sample course that is not restoring the homework grading category correctly. Note that the Exams grading category is correctly working.
Changing the aggregation type does not solve the problem.
The bug is a a little nasty in that you are unable to delete the category (it says it has a problem with setting the parent category) and you can not even delete the imported course fully for the same reason.
I have uploaded a screen shot of the original and restored course to demonstrate the issue.
This uses the latest CVS update of Moodle 1.9 Beta 4.