xterm has a secure keyboard option from its menu (control+click) that is supposed to prevent other applications from being able to receive X11 events related to input.
the thing you are referring to on os x is similar, with a system-wide capability. when you enter passwords for keychain and similar things, these have secure input enabled by default. i think it's up to the application to enable it, but when it's enabled for a field, no other application can intercept those events.
Thanks for pointing out the "Secure Keyboard" option. Make sure to test it before relying on it, though. I just tested in my system and it does not prevent "xinput test" from receiving the key presses, even when typing the "su" password.
right, because it's not operating on x11 events, it's just directly reading xinput data. there's not much xterm can do when the underlying server is giving away all of the raw input data.
the thing you are referring to on os x is similar, with a system-wide capability. when you enter passwords for keychain and similar things, these have secure input enabled by default. i think it's up to the application to enable it, but when it's enabled for a field, no other application can intercept those events.