Storing passwords to external services (e.g. corporate email servers) on smart phones is very insecure, since phones are more easily stolen. Has any vendor implemented a feature to only cache a password in memory for a limited amount of time? After the time period has elapsed, the app would ask for the password again.

EDIT:

I should've clarified - I'm aware that many (most?) users are lazy and want to just "set it and forget it". The always-remember feature will probably always be present. I was curious about an option to enable auto-forget for the security-conscious.