Know that problem? You are somewhere around and got special network settings like a wlan essid and static ip ressources. Your Gnome network manager which you really like as it finally brought easy wpa2/wpa/wep detection and setup to you prefers dhcp and skips around the networks like mad.
So this is the time to consider going back to the roots: Switch it off. Just disable the network manager by right clicking on it. Now open /etc/network/interfaces in an editor of your choice with sudo/root rights and add something like this: