Crystals and load capacitance
Check out http://en.wikipedia.org/wiki/Pierce_oscillator#Load_capacitance

The load capacitance is the total capacitance seen from the two pins of the crystal looking into the circuit.  So if you have a 6 pF from each leg to ground, that's 3 pF total (capacitors in series halve, capacitors in parallel add).

It's really rated for 6 pF?  That capacitance could be provided by the traces and PCB layout alone, I would think.  Typically I've seen more like 30 pF.