If you use locking, just look carefully at your Android phone screen
and you'll most likely already see the streak/smudge fingerprint
trace. An attacker only has to trace that in both directions and is
guaranteed access. By contrast, smudges left behind a PIN of N digits
offer N! combinations.

This is a common scenario that leaves the smudge easily visible:
1. Receive a notification of some sort (IM, SMS, e-mail etc.)
2. Unlock the phone (leaves the fingerprint trace)
3. Delete the notification (one or two taps that don't erase or
scatter the smudge).
4. Lock the phone (usually pressing a hardware button, leaving the
smudge intact).

Issue filed at http://code.google.com/p/android/issues/detail?id=3146#c4