A 040 additional info

In addition to the Dungeonscape Penetrating Strike alternative class feature, note there is also a Lightbringer Penetrating Strike ACF in Expedition to Castle Ravenloft (page 208). While the Dungeonscape ACF just says you get damage against a sneak-immune foe without qualifying that damage, the EtCR ACF gives you sneak attack damage with ½ the normal dice, and thus will qualify for the benefit of Complete Adventurer's Staggering Strike feat. As these are ACFs with two different names you can choose either one.