The best way (only-way perhaps) is to encrypt your sensitive data. A loopback device or a physical partition with LUKS should work well, as long all sensitive data are stored there. Encrypting the whole /home is not a simple thing for N900, so it could better work with the cooperation of programs that store sensitive data.

In any case, as a starting point, infrastructure for a password manager seems like a must these days. Encrypting the stored passwords is a first (and needed) step. I'm describing something like kwallet.

However, if you're considering the authorities, I suggest that you don't underestimate the point of the following sketch:

Indeed, that's why I said "simple tampering" Password-protecting the thing simply would not be good enough, though it too is better than nothing...

The best approach of course would be for the OS to provide function calls for reading, writing and creating encrypted files, and then for every application themselves to use those functions for any sensitive data and use non-encrypted function calls for nonsensitive data. That's how a well-designed, security-oriented OS would do it. But modifying every single existing application to use such functions would simply not be feasible :/

EDIT: Perhaps we could still petition for atleast Firefox and other large F/OSS applications to move to such design. The function calls for reading, writing and creating encrypted files could be made a separate library and thus shareable between several projects, and hopefully eventually become an inseparable part of any general distribution.

Ah, but in order to crack it, you need to find the encrypted password itself.
Which, in case you haven't been reading the forums recently, requires root access.
Provided you can use a password to prevent the raw data being read(i.e. lock down all methods of connecting before logging in), the only way to gain access to the raw data would be to physically remove the eMMC chip from the board and connect it to a test-rig.
Your average police officer doesn't have that. And can't do it in a short period of time.
Sure, if the FBI decides they want it, thet can try... But they'd probably just go with the rubber-hose method mentioned earlier.

In some countries the law says you must provide the key to encrypted data to authorities when asked, or else you get jailed, keep that in mind.