awesome idea. Honestly, many of my friends don't know anything about linux, hell, I don't know anything about linux. The main idea is that you want to lend people the phone, or they take it from the table to look through it. in that can you don't have time to tell them, man let me sign out and log you in as a guest. They just want to look around, and you don't have time to babysit them. Of course, if you see them sliding the keyboard and starting to type you realize that they might be in terminal so you slap them, but the average joe won't do that.

There are applications for this on almost every phone (default or aftermarket). I just want them not to be able to see my conversations, my emails and my pictures/videos from the GUI.

This sounds like a good idea to me - purely as I receive a daily text from my bank about cash coming into and (mostly) out of my account. I used to transfer these into a file on my n96 to keep a modicum of privacy but can't work out a way to do this on the N900. As a previous poster said I'm not after a military level of security just enough to keep casual eyes away

still nothing on this?

WIthout having looked into it at all, what if you:
1. Created a shell script
2. which you (an your friends) launch (from the desktop or app menu) instead of "Conversations"
3. the script would query for a password via some UI mechanism (e.g. zenity if that exists for Maemo?)
4. validate that, and then launch the actual Conversations tool assuming it's good.

This is in no way foolproof but it would stop the "just want to click to see what it is and -- ouupppss they did WHAT? OMG .." kinda stuff ..