Call recording has already been implemented - it's called "recaller"

Edit: Oops, in the few minutes I took to type up this post, someone already mentioned it.

x-term as root: apt-get install recaller

It would be quite a challenge to implement the functionality of an application like 'advanced device locks' in Linux (Maemo) - I don't think you're very likely to see such an application at all on the N900, though I'd love to be wrong about that. I think probably the only way to do it would be at the kernel level, but given how open the platform is, I would imagine it'd be fairly trivial to circumvent.

One word. Root.

Maybe you should investigate encryption - perhaps truecrypt might be of use to you.