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.