to quantify:
i always wanted to do top secret spy style fake thumbprint identification.

with flashing lights and awoogas if i get it wrong etc

Device locking desktop widget might be usefull for someone.

It shouldn't even be hard to implement since you can request screen blanking and keyboard/screen locking through dbus.

could the unlock screen be used in both landscape/portrait unlike now?

it'll be much easier to press the power button, swipe in portrait, then numerically enter lock code OR just enter a gesture (a la android) or something innovatetive and cool to then allow USING the phone functions directly.

This is 1 of the most imp things in my opinion because its a real pain to take phone outta pocket, fumble to unlock and then switch arnd to enter it in landscape and then be able to access the phone menu, hope it gets implemented screen because current way is not very convenient.

I believe a "Lock Code" like S60 utilizes should be included. It is simple and effected and could use an on-screen keyboard for 10-key.

Something a little more exciting, would be something I have recently stumbled on for use in FireFox...FireGestures.

Sure, Unlocking the device with a gesture is cool, but why stop there?

Gesture 1: Unlock device.
Gesture 2: Unlocks and starts camera application
Gesture 3: Unlocks and starts A
Gesture 4: Unlocks and starts B
Gesture 5: Unlocks and starts C

I don't have an N900 yet, but I wouldn't mind seeing this type of application be system wide even and be able to contain a lot of user-generated gestures for complete customization and control of the device, assuming there is no technical reason 50+ gestures couldn't be used!

I was just thinking about a gesture in front of the camera to turn the screen black and start something like flipclock as a panic button app in case you're surfing the internet and your boss comes by and/or picks up the phone. Could even be clap on clap off.

Gesture for camera already enabled, slide the cover open. works well as switching on camera whilst closed would result in very repetitive shots.

Panic Button: hit the power button once and screen defocusesmfor dialog box.

Imagine clapping every time your boss walked pass you

That's why I was thinking it should have multiple triggers. For example
1) clap
2) middle finger at the camera, just kidding. 4 fingers or ok sign
3) verbal word
4) can ir sensor trigger from hot coffee mug being held over it? Wait it's just a transmitter I forgot.
5) Sudden jolt from slapping the desk.
The idea is he doesn't see you touch it and whatever the trigger is it can be played off and varied. If he happens to pick it up and touch the screen turning it back on all he sees is flipclock.

Ooookay... stop kidding...

Just help to make this Application full customizable. LINK
Then you can use half press the Camera Button to access the Dashboard and if you want full press to access Flipclock instead of the desktop.

Or, if you want... full press of the camera button with closed shutter to lock your device.

The Button is Hidden when the Keyboard is opened. So noone will see that you're pressing a Button when giving the device away. (or something else you want)
Just my comment...

I think the accelerometer could be used for unlocking/locking the screen.

You would just shake the phone and it unlocks, or you shake and the swipe to unlock screen popups. The approximity sensor would detect when you have the device pocketed, so it wouldn´t unlock accidentally.

Locking by shaking could be done only from the desktop(s), so there wouldn´t be any accidental lockups when using applications.

Intuitive and simple.