I'd cut the traces on the Motor Shield going to 10, 11, 12 or 13 and re-wire them to other pins, like 5 or 6 for PWM and 7 or 8 for Direction.
The Motor Shield description shows the data pin connections as:
D3 -> PWM A
D11 -> PWM B
D12 -> DIR A
D13 -> DIR B
The schematic (
http://www.sparkfun.com/datasheets/Robotics/Ardumoto_v12.pdf) shows the four pins as being adjacent:
D8 -> No Connection
D9 -> No Connection
D10 -> PWM A
D11 -> PWM B
D12 -> DIR A
D13 -> DIR B
Gnd -> Ground
Aref -> No Connection
In either case it's a great way to guarantee that the motor shield is not compatible with anything that uses SPI such as your LCD shield or the Ethernet shield.