In the pre 9/11 days a colleague of mine lost her passport in the US and managed to fly home and enter the coountry using the 'personalausweis' (PA) the BGS or BP as it is now called let her in without any issues. The problem was with the airline staff, who wouldn't believe her the PA was equivalent to a passport in germany.
The official IATA rules say:
/ 03FEB07 / 1611 UTC
National GERMANY (DE) /Destination GERMANY (DE)
GERMANY (DE)
Passport (may be expired) or National Identity Card
("Personalausweis") required.
Temporary passport ("Vorlaeufiger Personalausweis") also
accepted.
Visa not required.
Minors:
Identity Card ( Kinderausweis , with photo) issued to children
up to 16 years old also accepted instead of passport (no photo
required for children under 10 years of age).