Very close, it's A4 & A5 with common ground and vcc.
so that's analogue 4 and analogue 5, thanks John. So when's the 10-core board design due...? 
I spent ages looking for this reference image last night, so I'll include it here:
[ http://www.arduino.cc/en/Hacking/PinMapping168 ]
If you look at the common duino boards with the USB connector pointing towards you, then the top 2 shield connectors correspond roughly with the pins as shown.
I was also referring to the Diecimila Schematic, but that doesn't include labels for SDA & SCL.
btw I suspect you may need an external power supply if connecting as many as 10 boards, since a USB port can only supply 500mA maximum.