Though this security warning applies to all touchscreen devices I believe it is a good idea to notify all N900 users about this:

People locking their device with a PIM code mostly unlock it again by using their fingers and the touchscreen.
Note that this action leaves traces on the display which make it easy to identify the most commonly used numbers on it so the propability of guessing the right code is dramatically increased thus rendering the code almost useless.
Using an adhesive strip to recover the fingerprint information is often enough.

Some advice:

• Use the pen to unlock the screen
• Clean the display regularly and (important) intensively. Traces will remain rather persistently.
• Use screen protector foil which is exchanged regularly (to be on the very safe side, cut it into pieces after removing)

As more and more private and sensitive data is stored on the devices some consideration regarding privacy and security is propably worth the small effort.

Or use a security code that uses all 10 number buttons!

And remeber that you don't need any codes to read pictures and other documents from phone. Even when you have the lock code in use..

Or simply don't store anything of importance on the device itself and grab it remotely

or just get yourself a life 007

I think ofels, has actually made a very very good point.

However, keeping your screen dirty, so you can't tell which buttons have been pressed anyway!