Arduino Uno R2 and Ethernet Shield R3 compatibility
The only issue that may create an incompatibility seems to be the IOREF pin next the 5V output pin which is extra on Revision 3 but not in Revision 2.

However, I just had a look at the board schematics and the shield does not use IOREF.


  


Pin 2 is extending IOREF

So, the shield should work correctly with the Uno Revision 2. As far as the extra pins are concerned, I would recommend not damaging them in any way. If you absolutely need to, then just try to bend them outwards very carefully.