Actually, it turns out that moving S20sendsigs to S45sendsigs is not a right idea because it somehow breaks the functionality of the following scripts on my second Feisty installation. Moving S31umountnfs.sh to S17umountnfs.sh seems to work better. The description in S15wpa-ifupdown says that it goes before sendsigs because sendsigs kills something it needs; in our case sendsigs must be killing network-manager which shuts down network interfaces which makes it impossible for CIFS to unmount its file systems properly.
Actually, it turns out that moving S20sendsigs to S45sendsigs is not a right idea because it somehow breaks the functionality of the following scripts on my second Feisty installation. Moving S31umountnfs.sh to S17umountnfs.sh seems to work better. The description in S15wpa-ifupdown says that it goes before sendsigs because sendsigs kills something it needs; in our case sendsigs must be killing network-manager which shuts down network interfaces which makes it impossible for CIFS to unmount its file systems properly.