In order to use the built-in Chat/E-mail, I have to save the password of my gmail account on N810. My question is, how is the password saved? If my N810 is stolen/lost, can the password be recovered by other people?

I'm assuming you're using diablo...
Get the configuration editor (gconfedit), and take a look at the folllowing:
For e-mail: /apps/modest/server_accounts -- You'll find another folder for each account you have. In those, you'll find the password for each account in plaintext.
For chavo (chat/voip): /apps/telepathy/mc/accounts -- again, you'll find folders for each account with passwords in, you guessed it, plaintext.

So.. don't lose it. Alternatively, set a lock code (and use "lock device"), which makes it considerably harder to get in. You'd have to know that reflashing it doesn't help, and there's some haking you need to do...

Thanks a lot for the info!

I'll just use a dedicated gmail account for n810... Locking code is too inconvenient for me.