I don't know why, but when i give my n900 to some of my friends, the first thing they do is enter the Conversations. Is there a way to put a password on it?

Nope! There is no difference on this than to handing your friends your phone when it does text and they go look at those. Maybe you should 'show' them your phone rather than giving it to them to have a look through unguided? Either that or delete conversations with private material prior to handing it over.

Obscurity is something different than security. Adding a password protection does not make it ANY safer at all. You just *feel* safer, but your data *is not*.
Lock your entire device instead with a password. Password "protection" (I can still access all the data by opening X Terminal, you are aware of this?) on a per-app basis does not make any sense.

I don't think he's asking for RSA 128 bit encryption here.. He just wants to stop people from reading his conversations. I doubt the people he is lending his phone know or even care about x terminal.

it makes perfect sense. He shouldn't have to lock down his entire phone in order to allow a friend to play a game or just use the phone.

Why should one make his own life harder as it is by locking away essential functions on the phone? If your friends don't respect your privacy, just don't hand them your phone. If they just want to do a call: let them go to the next phone box. If there is no: bad luck.

Me, I would NEVER shoot myself in the foot by locking down my phone just because some curious guys can't keep their eye from my private data. And I have no problem to tell them.

Why don't you post a few spicy excerpts here and we'll let you know how you should protect them?

Damn. I wish I had something in my conversations worth keeping hidden.

I don't think I have ever seen the messaging application of ANY phone have a password lock. Is there anything wrong with simply telling them not to open your conversations app when you hand the phone to them?

As long as you have any conversation, you have something worth keeping hidden. It's a question of privacy, data security and informational self-determination.

As soon as you say: "I have nothing to hide", you open the doors for big brother and all his brothers and sisters.

I could put a lock on messages on my old samsung G600 , you could put a lock on most things such as files and games as well. But the lock on messages was very useful as it thwarted my pesky little brother in his efforts when trying to get a sneaky peak at my texts!