Docedison:
Where is it on the "REV 3" schematic? I've looked and searched for a pin marked IOREF and I can't find it although there's a Pin clearly marked IOREF..Doc
They failed to label the two new pins added to the Uno rev3 board, but they did append two new pins onto the shield 'power' connector that now has 8 pins Vs 6 pins on older arduino boards. The new power pin1 is reserved for future use. Pin2 is the IOREF pin (you can see it's wired to +5vdc) and is used to tell any shield attached what voltage the host arduino board is running at, so that future shield boards can be designed to be able to be compatible with 5vdc arduino boards or the future Due 32 bit board which will run at 3.3vdc. Here is the wording they used to explain the new additional pins added.
Revision 3 of the board has the following new features:
1.0 pinout: added SDA and SCL pins that are near to the AREF pin and two other new pins placed near to the RESET pin, the IOREF that allow the shields to adapt to the voltage provided from the board. In future, shields will be compatible both with the board that use the AVR, which operate with 5V and with the Arduino Due that operate with 3.3V. The second one is a not connected pin, that is reserved for future purposes.
Stronger RESET circuit.
Atmega 16U2 replace the 8U2.