A shifter (Races of Eberron 25–40), in addition to the feats Endurance (Player's Handbook 93) and Toughness (101), takes the feats Shifter Stamina (115–16) and Troll Blooded (Dragon #319 61).1
The extraordinary ability regeneration 1 that's granted by the feat Troll Blooded causes the shifter to be dealt—all the time—nonlethal damage except by effects that deal acid or fire damage. The feat Shifter Stamina renders the shifter—while shifting—immune to nonlethal damage. Hence, while shifting, unless an effect deals acid damage or fire damage, for the shifter, the effect deals no damage.
However, I'm unsure if this also renders that shifting shifter immune to any ancillary effects of a damage-dealing effect, like a poison were the poison to have been delivered by a normal weapon attack or the ability damage that would've been dealt by the rogue's special ability crippling strike (PH 51).2
Is this immunity to damage defender-facing, as in That effect dealt me no damage therefore I ignore all of its effects completely? Or is this immunity to damage attacker-facing, as in O, I totally dealt damage with the effect, but the enemy to whom I dealt it just so happened to be immune to it? Or some combination of both? Or is there another—I hope easier!—way of adjudicating this combination of abilities?
1 The Troll Blooded feat's regional type requirements can be met with 2 ranks in the appropriate Knowledge skill. Greyhawk regional feats have rules for later-than-level-1 acquisition similar to how Forgotten Realms regional feats functioned when originally introduced in the Forgotten Realms Campaign Setting. While how Forgotten Realms feats could be acquired was changed by the Player's Guide to Faerûn, nothing changed how Greyhawk regional feats can be acquired.
2 For comparison, the Dungeon Master's Guide glossary on damage reduction, in part, says, "Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury type poison, a monk’s stunning, and injury type disease" (292). To be clear, a similar note is absent from any description of the regeneration ability.