If I had a child I'd want to be able to let them use my phone but only in a special mode that allowed access to a limited number of whitelisted websites & apps.
http://www.schneier.com/blog/archives/2012/07/all-or-nothing... http://cups.cs.cmu.edu/soups/2012/proceedings/a2_Hayashi.pdf
But: yes.
I'd like my phone to offer a few different "shells" of access:
- Emergency calls.
- "Share" or "play" mode. Selected photos or apps.
- "Mobile" -- ready access to stuff I need, but not distractions (the "Car Panel" on some Android phones is somewhat like this, but I'd appreciate if it didn't encourage use while driving).
- "Full" complete and potentially immersive access.
That said, it would be nice to have the option of telling the app to cache your credentials.
A short appstore PIN could solve this much more easily.
Now, an SD card with a certificate plus face recognition might be ok ;-)
But for biometrics, keep in mind that biometric systems are currently seen as the most subject to false positives of any authentication system out there with the possible exception of improperly maintained and insufficiently strong passwords.