With the new update IOS6 yes there is a problem with the software. After i replaced a screen on a 4s that had this new update i too got the searching and after pulling the phone apart 4 more times i had no idea what eles to do.
So after the last time i put the phone together with the sim in and got the searching again I picked up my phone and called the mobile... Not sure why, but it worked and i then got reception.
I have repaired 5 phones since then with this IOS6 update and all of them had this searching and i just called the phone.
Hope this helps others.
Hope it make sence :)
Just call the mobile number on the phone that your fixing