The pushback duration reduction from this talent stacks with other similar effects, such as Earth Shield and Concentration Aura. The effects are additive, and it is possible to reach 100% pushback reduction. Combining three points in Healing Focus with Earth Shield will make the shaman immune to pushback.
Note that pushback is not the same thing as interruption. Healing Focus will not give any protection against abilities such as Kick or Counterspell. Instead, the Restoration talent Focused Mind reduces the duration of such effects.
/Patch 3.0.2 (14-Oct-2008): Reduced from 5 to 3 points. Changed from giving a chance to ignore pushback to reducing the duration of pushback effects.