iPhone 3G Battery
I totally agree with Andrew, when changing the power and sensor cable, do not do this step. It is absolutely unnecessary and only increases the pain when reassembling your phone. That step should not be in the guide for the power cable replacement.
When I had changed the dock connector, my iPhone was not able to get a carrier signal. I found an helpful answer in the forum which worked perfectly for me. Make sure you set the correct date and time (after you remove the battery, it will most probably show you 1970) remove the sim card and plug it in again. That worked for me.