The text for smite evil (and other smite options) states that the paladin’s melee attack that gets improved by smite “deals 1 extra point of damage per paladin level.”
Nothing modifies or caveats this damage as far as its type is concerned—whatever type of damage you were dealing, you add your level to that as a bonus. Attack with a longsword, add your level to the slashing damage you were already doing. Attack with a mace, add your level to the bludgeoning damage you were doing.
The question came up elsewhere about whether or not this extra damage could be nonlethal. That is, if you used an unarmed strike or sap, or chose to take a −4 penalty on some normally-lethal weapon’s attack, so that your melee attack is dealing nonlethal damage, does that also include the extra damage from smite?
Officially, the answer seems to be yes—the primary description for smite is, ya know, smite, and it just says to add a number to your damage roll, nothing about types of damage. Smite doesn’t have the statement that, for example, sneak attack does, which says a rogue “cannot use a weapon that deals lethal damage to deal nonlethal damage in a sneak attack, not even with the usual −4 penalty.”
But it still seems to be something that players and DMs find weird; there seem to be many tables that assume smite always deals lethal damage, even though the text doesn’t say that. For that matter, the Holy Subdual feat from Book of Exalted Deeds offers the “benefit” of “When you use your weapon to deal nonlethal damage, you can also transform bonus damage from [...] smite evil [...] into nonlethal damage,” which implies that without the feat, you could not do that, suggesting that even some WotC authors (not their best, I’ll grant you, considering the overall quality of Book of Exalted Deeds) believed that smite evil’s extra damage was always lethal.
So is there anything out there that would give this impression? Is it perhaps a holdover from 3rd edition, or even earlier editions (Book of Exalted Deeds does kind of occupy an awkward “3.25” space, having been written during the transition—you might note the use of the term “subdual” that 3.5 replaced with “nonlethal”)? Are there descriptions in any of the books that suggest an inability to use smite evil nonlethally? I’d even be interested in a D&D novel where a paladin couldn’t smite nonlethally (provided it is a distinctly D&D novel with a distinctly D&D paladin distinctly using smite).
I am looking for explicit mentions of smite evil always being lethal (or explicit statements that it’s not always lethal and can be made nonlethal in the usual ways), from official sources (WotC published or WotC approved, or TSR published/approved if claiming the notion is a holdover from prior editions). I might accept an answer that can really provide a convincing account of “this is just how everyone always played,” but it would have to be extremely well-documented and backed up to convince me. I explicitly am not interested in whatever you or your tables has done.