PWM above 16v for speed control? Will it work?
Shoot-through current refers to the condition where both switches/MOSFETs on one side of the H-Bridge are on simultaneously.  Under normal conditions, the H-bridge is under one of the following conditions:  

  

If, however, both switches on one side are on simultaneously, a huge current can flow (Only 34mΩ per leg, remember?), which is usually destructive.  I'm not sure why both switches would be on - Possibly an issue with gate capacitance and switching time?

The datasheet says that the PWM pin is:


  Voltage controlled input pin with
  hysteresis, CMOS compatible. Gates of
  low side FETs are modulated by the PWM
  signal during their ON phase allowing
  speed control of the motor.


You might get better luck with higher voltages if you modulated the PWM such that it was centered on the ON-pulses:


Motor on:     ----    ----     --      --
S-Left-Top__/------\________/------\________  
S-Rght-Bot___/----\___________/--\__________
S-Rght-Top__________/------\________/------\
S-Left-Bot___________/----\___________/--\__  


But you'll definitely want to look very carefully at Figures 4, 5, and 6 of that datasheet.