This error can also happen with the Duemilanove when you have the RX0 port of the Arduino connected to a TX port of a device such as a motor controller. It can be fixed by removing the RX connection. If you need feedback from the device on the RX connection then you may want to install a switch between the device and the RX connection of the Arduino that can be turned off when the USB is connected.
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x00