The Shield Base doesn't support SPI or I2C in its standard configuration quite yet. We're getting close to SPI and UART; I2C will come soon after--and you'll be able to use pins A4/A5 or SCL/SDA. They're mirrored.
In the most recent copy of the shield base assembly, I've noticed that the I2CDevice function is still not implemented. Is this tied to 4.3 as well by chance or still far out?