Can't set Contact Subtype from Batch Update Via Profile

This problem will slow take up of the subtype feature, as it prevents contacts being changed in batch to take on the new subtype.

This problem is reproducible on both 3.1.beta1 (Joomla) and the sandbox (Drupal), by the following steps...

1 Create a subtype
2 Create a profile with one field: Contact > Contact Subtype
3 Run an Advanced Search to return a number of contacts whose type is the parent of the new Subtype
4 Select all or some of the contacts and choose the action Batch Update via Profile
5 Select the new profile
6 The drop-down for Contact Subtype is empty (ie, one entry saying '- Select -')