Does not affect summon creatures; the enchantment shows, but does no actual fire damage. In the Mac version, however, undead wielding melee weapons (summoned via Animate Dead) do benefit from the spell.
The damage bonuses are determined when the mage casts the spell; changing equipment while the spell is active does not change the damage bonuses.
Per the mouse over description in the name bar, only affects melee weapons. Bows and staves do not get a bonus.