This feature was requested by Timothy Takemoto.

See this forum for more on this topic.
http://moodle.org/mod/forum/discuss.php?d=50063
Ken Wilson has written code for this feature. See attached zip file.

From Tim:
It would be nice to combine [the feature] with a method of limiting the number of courses that students can sign onto. Perhpas within the course their might be a setting

"Only allow one course registration to courses within category..."
followed by a drop down box showing the categories (and parent, and grand parent categories) that the course belongs to.