Related, but slightly off topic - anyone have a link or quote on the ruling that weapons that change an energy descriptor (ex: Flaming weapon) are additive on attacks that have an energy descriptor (ex: Split the Sky)? I could've sworn I read something WotC-ish, in a book or FAQ, to that effect, but can't recall where I heard it...
It being one of the most common ways to get two mingled damage types on an attack, in theory, if I'm not off my rocker on reading that.
You might be thinnking of the Keyword Issue mentioned in the FAQ - It says that the attacks effectted by a Flaming Weapon gain the Fire keyword if the power with the Fire keyword is in effect at the same time as the attack power.
Thus if you had a Weapon Implement and used Acid Arrow with a Flaming Weapon using it's At-Will power you would do have an attack with both the Acid and Fire keywords (so it counts as an Acid and a Fire power for feats/effects that effect either).
However this only applies to the keywords. The damage in the above case would be fire damage - All damage dealt by the Flaming Weapon is fire damage when the At-Will power is in effect. This overwrites the basic type of the damage. This is indicated, imo, by the "Another free action returns the damage to normal" thus indicating the 'normal' damage type stopped being true. This would also explain why you would turn off the effect sometimes - as otherwise the damage is always fire and ..... damage which is always better, especially if the ....... was untyped (as untyped damage is never going to be a bad thing - as the only thing that negates it is Resist All, which will negate any typed damage too).