The 3G board with A4 processor.

The second shot is the main board from the Wi-Fi iPad.

Key differences?

The most obvious change is the addition of the connector on the right side of the board for the communications board.

More subtle is an additional small IC above the A4 chip next to the connector for the GPS antenna. This may be a signal processor, but we can't firmly identify it: T3J 927 632567.