You fail to describe what problem you're trying to solve (apart from "I have nude pictures of my girl on it. *blush*") and from whom you want to hide your home made pr0n.

Identify the attacker first and then look for solutions. I'll make a list and you can figure out what you actually want:

- Nosy "friend", no Linux knowledge
- Nosy "friend", with Linux knowledge
- Random person who finds your lost device
- Serious person who finds your lost device
- Serious person who steals your device
- The government (in the shape of border control)
- The government (in the shape of the police)
- The government (in the shape of a special branch)

Now to solutions:

- Lock your device by pushing the power button and selecting "Secure device"
- Place your sensitive data in a not obvious place
- Create a directory with a name starting with "."
- Place your data in an archive (zip, tar.gz, rar)
- Place your data in a password protected archive (zip)
- Place your archive in a not obvious place
- Encrypt your data (PGP, GPG)
- Create an encrypted file system as a file to be mounted and save your data there
- Create an encrypted file system as a partition that has to be mounted and save your data there
- Create an encrypted file system inside an encrypted file system to create some kind of plausible deniability

Also, keep in mind that the N900 wasn't designed to be a spook tool. SMS messages are stored in a data base and to hide some messages, you'll need to, as far as I know, hide them all. Images are indexed for a reason so trying to hide them goes against the design.