I agree on using a password protected archive as it might be a good compromise. This would then make the password into the "lock", which might very well be a PIN (as in numbers).

This would work on files, but since SMS messages are stored in a database, things get a bit more complicated with those. A suggestion might be to try to extract them from the database and have a separate viewer for those extra sensitive messages. These messages could then be saved in their own database that's in turn protected in an archive as any other file.

For files:
First, pick an archive format that's competent enough and, preferably, has been ported to the N900 already.

Secondly, flesh out the details of how to select files to protect, where to store them and how passwords should be handled.

Thirdly, get on building a GUI.

For SMSs:
First, have a look at the SMS database that's used.

Secondly, try to find out if there is an API to be used to extract the messages.

Thirdly, decide on how these extracted message should be presented when you want to look at them.

Fourthly, integrate this into the GUI from the previous TODO list.