Master of Flame paragon path has a level 12 Utility Power called Burning Transformation.
Burning Transformation - You transform into a creature of fire until the end of the encounter. While in this form you gain the following benefits and limitations: You are insubistantial, you can shift 1 square as a minor action, you are immune to fire, you gain a +5 power bonus to damage rolls with arcane fire attacks, and you cannot use any attack power without the Fire keyword. Aditionally, whenever an enemy hits you with a melee attack, it takes fire damage equal to 5 + your Intelligence modifier.
That said, Could a Swordmage wielding a Flaming Bastard Sword (with the Fire damage activated)use any Arcane Attack Power, for example: Sword Burst, as a power with the Fire keyword? In short, while Burning Transformation is active you can use any offensive Arcane attack power because a Flaming Weapon grants them the Fire keyword?