I checked a bit what I can find in Longhorn concerning that Biometric stuff. And I found that it is BAPI: bapi.dll, . That specification was bought some long time ago from i/o software and I remember that it was planned to have this integrated in Windows XP. I think that Microsoft did not included it because the biometric technology was not ready at that time.
I know a bit more about BioAPI than BAPI, but at the end I think it works almost the same. There is also a Kernel Biometric Service Provider: kbsp.dll and a tool called biotool.exe permiting to enroll a new biometric credential:
BIOTOOL <command> <method> <source>
ENROLL Enroll a new biometric credential.
sources (specific to each method):
RT Right Thumb (FP)
RI Right Index (FP)
RM Right Middle (FP)
RR Right Ring (FP)
RP Right Pinky (FP)
LT Left Thumb (FP)
LI Left Index (FP)
LM Left Middle (FP)
LR Left Ring (FP)
LP Left Pinky (FP)