After much head-scratching, I am considering ruling that an invisible wizard casting flaming sphere does not cancel the invisibility. Does anybody have a take on this?
Consider: flaming sphere's "effect" is the sphere itself, which moves at the caster's command. There is never a "target;" if it enters a space with a creature, that creature must make a Reflex save. This implies to me that a FS is an environmental hazard more than a weapon. If summoning a monster doesn't cancel invisibility (and I consider that a stretch), then FS certainly doesn't.
Consider: flaming sphere's "effect" is the sphere itself, which moves at the caster's command. There is never a "target;" if it enters a space with a creature, that creature must make a Reflex save. This implies to me that a FS is an environmental hazard more than a weapon. If summoning a monster doesn't cancel invisibility (and I consider that a stretch), then FS certainly doesn't.