How secure are the "app password protection" apps? I know there are a number of apps available that can password protect specified apps. I want to know how secure they really are.

As far as I know, there aren't any exposed OS hooks in android to allow one app to prevent another app from starting up. And some of these apps can be fooled by just switching tasks when it presents the password screen. So, are all of these apps just for casual protection, or are there any apps out there which guarantee that password protected apps can't be launched without the password?
Those protected apps aren't secure at all. These protects are meant for girlfriends only... :)
As you've said Android doesn't allow one app to prevent another app from starting up, there's no way to really protect an app.
The password screen can be by-passed by disabling one receiver of protector app. Even its also not required for some poorly written protector apps: Just, kill the protector app and done! And, the most easiest method is more funny: Many protector apps provide a failsafe password to by-pass protection in case user has forgotten the password. You just need to contact developer saying you have forgotten your password &amp; he'll provide you a failsafe password. :)
If your device is rooted, I recommend you to use Seal. Seal provides a root method which is much more secure than generic methods. Disabling a receiver doesn't by-pass its security. As it gets root permission, it heal itself from any attempt to by-pass its security. So, even malicious codes fail against it.
In the last, no protector app encrypts an app &amp; decrypts it on fly before opening it. Its simply because Android system simply doesn't support it. So, an advanced user can access your protected apps &amp; associated data using Android SDK.