If you get following error message with xcode 4.2 and trying to compile and run in iPhone 3G or older itouch, you need to add “armv6” in your build settings under “Architectures.”

unknown lockdownd[16] <Error>: 2ff68000 handle_connection: Could not receive USB message #7 from Xcode. Killing connection
unknown com.apple.mobile.lockdown[16] <Notice>: Could not receive size of message
unknown lockdownd[16] <Error>: 2ff68000 handle_connection: Could not receive USB message #6 from Xcode. Killing connection
unknown com.apple.mobile.lockdown[16] <Notice>: Could not receive size of message
unknown com.apple.mobile.lockdown[16] <Notice>: Could not receive size of message
unknown lockdownd[16] <Error>: 2ff68000 handle_connection: Could not receive USB message #6 from Xcode. Killing connection

Also make sure you have your deployment target to appropriate ios version under the “info” settings in your project.