The Bundesrat issued the 18th September 2009 the new administrative instructions of the Residence Act (Aufenthaltsgesetz amended in 2009), which § 87 concerns the obligation to denounce and § 88, the interdiction to denounce the persons without a residence permit to the foreigners office.
According to the new instructions, the prohibition to denounce is extended to the welfare services which are contacted by a hospital for a refund (only hospitals are explicitly mentioned in the text). However, if a person without a residence permit contacts directly the welfare services, the reporting requirement is not removed.
Check pages 507-509 of the Residence Act for these new instructions (pdf document in german only).