It's not the same sensor. Canon's web site describes it as a new design.
Of course it's not the same - it's 20 MP, not 22 MP. But if it uses the same production technologies found in the 5DIII and 5DII, the IQ should only be marginally different, if at all.
I was holding the 6D in my hands and asking the Canon-representative here in Sweden about the sensor and especially the low-light-capacity and
he (K. Mörk, Svenska Canon Sweden) told me that this sensor is NOT the same as used in 5DMK3 but it is totally NEW and that he was thinking that 6D sensor will be performing a bit and slightly
better than even the 5DMK3 but this said in the low-light-situations when using high ISO. This is from our conversation from 31st October 2012 - that is my two cents in this thread)
Have a good day!