Is a Smite Evil attack considered magic or holy? Does it bypass enemies with neutral damage reduction?
Smite Evil says, in its entirety (aside from some explanatory notes about invalid targets and uses per day as you level up):
Once per day, a paladin may attempt to smite evil with one normal melee attack. She adds her Charisma bonus (if any) to her attack roll and deals 1 extra point of damage per paladin level.
Nothing in that description says anything about the attack being magic or holy, nor does it say anything about bypassing DR of any type. Searching through the d20srd.org site for Smite Evil returns nothing to counter that.
As written, Smite Evil does not inherently bypass any form of DR, nor does it inherently count as Magic or Good for any purpose. Of course, the weapon through which the smite is delivered would bypass DR as normal.
Note that the Pathfinder version of Smite Evil does bypass any DR that the (valid) target might have. It's also more powerful in several other ways.
A creature with this special quality ignores damage from most weapons and natural attacks. The creature takes normal damage from energy attacks (even nonmagical ones), spells, spell-like abilities, and supernatural abilities.