Staggerin Strike (feat, Complete Adventurer, p112):
If you deal damage with a melee sneak attack [...] your target is treated as if it were staggered, even if its nonlethal damage doesn't exactly equal its current hit points [...] This feat has no effect on creatures not subject to sneak attack damage.
Grave Strike (spell, Spell Compendium, p107):
[...] you can deliver sneak attacks against undead
I could read it either way:
- Not staggered: I did deal SA damage in this particular case, but the creature is "not subject to sneak attack damage"
- Staggered: The spell completely negates SA immunity for this attack
Can Grave Strike help to make an undead staggered?