$ VBoxManage modifyhd name|uuid --autoreset on|off
command will have no effect. What's worse, it seems that the affected versions of VirtualBox are sticking with an autoreset value set to off.
This basically means that, after stopping and starting a virtual machine, the differencing disks won't be wiped off automatically hence changes made into your guest instance will be persisted upon reboot. This defies one of the common use cases where VirtualBox immutable images and differencing disks are most useful.
If you don't know what immutable images and differencing disks are, keep in touch. A future blog post about these features has already been scheduled.
Update: I verified that VirtualBox 3.2.8 is not affected any longer by this bug.