Been doing some Mac Mini quad core research.
I believe 2013 for the following reasons...
All the standard level Intel quad core processors have a TDP of 45w. At its peak the highest wattage Intel processor in a Mac Mini has been 34w. It's now 25w.
More wattage means more heat and a bigger fan to move it. Apple have been shrinking the Mac Mini.
There are no Ultra Low Voltage versions of Intel quad cores, right up to Sandy Bridge designs. The Ivy Bridge presentation said that TDPs would stay around the same for quad cores.
Apple could make a design with a thin height full width fan to accomodate for the heat. This is very unlikely as 45w quad core chips were around when the half height Unibody Mac Mini was released.
So that leaves a different manufacturer of the chips..
The Apple moving to ARM rumour would make sense. They have in-house designs for the A4 and A5. The base A6 could be quad core (based on the ARM Cortex-A15) but that would only be for iOS devices as it's 32bit / 40bit.
ARM have said that 64bit Cortex is a possibly but gave no more details. It is unlikely to be within the A15 design.
My prediction is the Apple A7 will be 64bit quad core. As 64bit has the memory addressing it can be used in the higher spec Macs. OSX 10.8 would carry support.
Windows 8 will have 32bit ARM support. Will Apple back track and let Lion have 32bit support?
Time will tell...
Research
ARM Cortex-A15
ARM CEO on 64bit
Windows 8 on ARM
TDPs
Legacy Quad Cores
2GHz Q9000 TDP 45w Core 2 Quad
2.26 GHz Q9100 TDP 45w Core 2 Quad
Mac Mini 1,1 Early 2006
1.5GHz T1200 TDP 27w Core Solo
1.66GHz T2300 TDP 31w Core Duo
Mac Mini 1,1 Late 2006
1.66GHz T2300 TDP 31w Core Duo
1.83GHz T2400 TDP 31w Core Duo
Mac Mini 2,1
1.83 GHz T5600 TDP 34w Core 2 Duo
2.0 GHz T7200 TDP 34w Core 2 Duo
Mac Mini 3,1 Early
2.0GHz P7350 TDP 25w Core 2 Duo
2.26GHz P8400 TDP 25w Core 2 Duo
Mac Mini 3,1 Late
2.26GHz P7550 TDP 25w Core 2 Duo
2.53GHz P8700 TDP 25w Core 2 Duo
Current Mac Mini 4,1
2.5 GHz P8600 TDP 25w Core 2 Duo
2.66GHz P8800 TDP 25w Core 2 Duo
Latest Macbook Pro
2.0 GHz 2635QM 15" TDP 45w Core i7 Quad
2.2 GHz 2720QM 15" 17" TDP 45w Core i7 Quad
2.3 GHz 2820QM 17" TDP 45w Core i7 Quad