A few notes about how I got working fprint in Ubuntu Karmic (since all available HOWTOs treat old versions, other distros or other fingerprint frameworks).
auth sufficient pam_fprint.so
# here are the per-package modules (the "Primary" block)
That's it: when you reboot, you should be asked to authenticate via the fingerprint reader (and only if it fails via password).
So I personally think that fprint is at the moment quite useless. An exception may be its usage for sudo/gksudo/policykit authentication.