In symfony 1.0, you could set the session timeout in the apps/*/config/settings.yml file, with the sf_timeout setting. This is not used anymore. In symfony 1.1, this must be done in the apps/*/config/factories.yml file, by changing the parameters of the user factory. This does not appear to be carried over by the project:upgrade1.1 task, so you must do it yourself. For example:

[yml]
all:
  user:
    class: myUser
    param:
      timeout:     1800     # session timeout in seconds