I think many people will benefit from this application of app-by-app lock.
When an application is clicked e.g. Messages, a password or pattern screen pops up.
There is an application for that on the iPhone (jailbroken) called Lockdown.
[RELEASE]~Lockdown v.5.0, password protect your applications! - iPod touch Fans forum
So anything similar with that concept in mind would be great!