Flaming whip, burst alight!
I've read most of the posts in this thread, and I have to say it was an interesting discussion, and very very amusing. Yet there is one thing I believe hasn't been covered. So sorry folks, I'll bring back this issue. And if you know any FAQ or errata that says something decicive on this, please give me a link.
Anyway, PHB, p. 114: "Weapons are classified according to the type of damage they deal: bludgeoning, piercing, or slashing. ... Some weapons deal damage of multiple types. ... If a weapon is of two types, the damage it deals is not half one type and half another; all of it is both types."
Fire is not mentioned, so weapons cannot deal fire damage. The list is restrictive, there is no indication that the three types serve merely as examples. (That's just the literal reading of this passage, leaving no room for interpretation) Also, it is said that a damage dealt by a weapon of two types is of both damage types. So, if you go as far as to believe that a +1 Flaming longsword deals two types of damage, you'd have to accept that a troll hit with that weapon suffers all of the damage as fire damage. But we all know it's not part of the slashing damage - the only type of damage a longsword is capable of dealing. Sneak attacks, str bonuses, enhancement bonuses and such add to the slashing damage, fire is energy damage. As someone else proved, the energy deals the damage, how else could ranged weapons bestowthe energy to ammunition?
It could also be said that PHB does not cover magical enhancements, so the DMG rules override the PHB rules. Also, the whip is "slashing" and does not damage armored opponents. The PHB does not cover situations where the weapon has an enchantment that deals damage in addition to the slashing damage.
Still, I'd like to see some official ruling..
And there is no such thing as nonlethal fire. I can't recall any reference to such damage anywhere in the core set. For example, environment dangers deal "nonlethal damage" not "nonlethal [energy type] damage". Please, show me where it is said that something deals "nonlethal [energy type damage? "Should" means it is at DM's discretion to decide otherwise. DM's discretion is in effect using house rules. For example, the area of "Entangle" can vary, at DM's discretion, depending on the terrain. Since no rules for different terrains are given, it's a house rule.