First, check that you didn't leave any code (from testing maybe) that can log out the user. Also, does the user gets logged out after 2 minutes of being not active?, because that is normal behavior.

I had the same problem, it was problem with the session file system i switched to memcached and the problem has been solve for me

@Ruk33: Why are 2 minutes of not being active normal behavior? It is set to 120. But the comments (in session.php) are saying, that this is the time in MINUTES and if I take a look at the Cookie Tab in Firebug, the Session expires after 120 minutes.

@fenos: I'm using the database driver. We don't have memcached on our deployment server.