I've already filed a bug: http://dev.rubyonrails.org/ticket/10804 I'm still not sure if i'm missing something.
When I update a child by setting the association method to nil, the counter of the parent does not get decremented. For a code example see the first answer to my ticket.
The problem seems to be that the @owner variable in activerecord/lib/active_record/associations/belongs_to_association.rb holds the child object, not the parent. That's why it cant have the counter_cache_column and the test: if @owner[counter_cache_name] and not @owner.new_record? fails.
I really would appreciate anyone who gives this a try with his installation and reports the results. The original bug (and a patch) about this issue has been posted about 2 years ago!
_jeanmartin