It won't have the same impact unless MS wants to give away the Arcade SKU at $99 (bare console, no Natal) and they drop the price of the 120GB SKU to $199 (no Natal).
More likely scenario: MS includes Natal with the Arcade SKU for $199, 120GB SKU goes to 250GB with Natal for the same $299.
Is the second option a great bundle? Absolutely. Will it have the same impact as the PS3 dropping to $299? No.