This is one episode that everyone should watch or listen to

Session hijacking. In short, by using tools available now, you can effectively
1. Use Jasager [http://www.digininja.org/jasager/index.php] to let people think that they are on their default wifi network. Meaning, i go to a starbucks, i turn on my wifi, and my laptop automatically tries to connect to wireless@SG. Jasager lets you log on to it, making you think that you have connected to the wireless@SG hotspot. You start surfing, thinking cool, free internet again
2. Prankster, hacker, kid, whatever who had setup the jasager, use ferret and hamster and tap into whatever you are surfing, using you session, your cookie, etc. http://erratasec.blogspot.com/2007/08/sidejacking-with-hamster_05.html
3. Game over, you find you have posted some things in facebook or started to poke people at the wrong place, started to mass mail your friends, etc.

From point 2, you can see that this is a very old post, so i hope that this post will bring about more awareness.
So how to prevent this from happening? You can’t really, you just have to be careful, you have to know the infrastructure of the network like what Hak5 said.
1. For example, if you are logged in to your home router ssid, but hey you are at macdonalds, that can be right? You better get your butt off from that network.
2. For example, if you are logged on to wireless@SG, it did’nt prompt you for the usual password? Something is wrong, even if it prompts you for your password, better check the SSL certs, etc to make sure you are on the right wifi network
3. Make use of SSH tunnelling, so that people won’t be able to see the packets and side jack you.

The video just blows me away, you got to watch it, to know how bad this is.