You can launch steam normally, then choose to restart it into offline mode, (steam menu, go offline) which it will stick in until you turn it off, or reinstall, or something. (though every time you start it up it asks to make sure you want to stay in offline) That's about all you get.
Again counter intuitive to have you log-in online via internet and verify your account before you can play offline when the entire reason you want to play offline is because you lack internet. Very counter-intuitive if you ask me.
I can't predict weather or not I will know the local WEP keys and I like to maintain online when I can to Steam chat with people I know so when your need to game offline is unpredictable Steam makes it very inconvenient.
I might try Rincewind's method to see if it works but either way Steam makes "offline" gaming a hastle compared to Impulse, D2D, or GamersGate where the latter two don't use specially wrapped .EXEs for their service.
I love Steam to death as I am always playing TF2 or DOW2 both of which require steam and many of my friends chat online using Steam's IM service but the way they handle offline gaming continually bugs me.
I would prefer if they were a bit more like Impulse which doesn't bat an eye if you don't have internet and just disables internet-required functions if no connection is found instead of having an entirely separate mode you have to log into, verify, and boot to when you are without internet.