This is totally inaccurate. The memory clock speed in the 2011 minis is 1333MHz. You can go down but not up. Putting 1067MHz RAM in a 1333MHz slot will result in the memory not working, because it cannot reliably run at 1333MHz (otherwise it would have been sold as 1333MHz RAM). It probably won't fry the RAM but it simply won't boot. Being able to overclock RAM by 25% in clock speed is virtually impossible these days.
OTOH, you can put 1333MHz RAM in a 1067MHz slot and it will work just fine. In fact, some of the later C2D macs that shipped with 1067MHz RAM can actually run 1333MHz ram at the full speed. My 2009 13" MBP does this.
Ruahrc
And, FWIW, a 25% increase in RAM speed may not be the most obvious upgrade in day-to-day usage, but it definitely runs faster. So if your Mac shipped with 1067MHz but supports 1333MHz RAM, there would be no reason not to pay the couple extra dollars' difference.