I created two simple test Person Accounts. I execute the merge method in my code and get the following error:

Merge failed. First exception on row 0 with id 001e0000008raxdAAA; first error: INVALID_FIELD_FOR_INSERT_UPDATE, Unable to create/update fields: PersonContactId, CreatedDate, IsPersonAccount. Please check the security settings of this field and verify that it is read/write for your profile or permission set.: [PersonContactId, CreatedDate, IsPersonAccount]

It would seem that is choking on the read-only system fields of which would never be writable. This would imply that merging person accounts doesn't work at all. Has anyone gotten this to work?