That's a 2.4GHz C2D machine with a 320M GPU, right?
The 13" MBP and MB of that generation with the same CPU/GPU would not correctly clock down RAM (and I believe the Mini has roughly the same, if physically reconfigured, motherboard), so it had to be 1066MHz. I ran into this (and researched it extensively) when I tried to use the 8GB 1333 RAM from my 15" 2011 machine in my wife's 2010 machine. The RAM was without question, known, perfectly working for a couple of years (then continued to work when I reinstalled it in my 15").
Funny enough, if you used a single 1066 DIMM combined with a 1333 DIMM it ran fine, apparently the 1066 RAM would cause the system to forcibly clock down (I ran her machine with one of each 2 + 4GB DIMMs and it ran perfect at 6GB total, 1066MHz).
Some RAM has an unlocked, programmable clock, so with some voodoo and command line apps on a PC you can change it, but that's generally not for civilians