no its not. for all you know Microsoft will make XBL Online Free on the Xbox 720 to compete directly with Sony's Free PSN.
i will laugh really hard at those people who stack up on these cards only to find out in 6 Months that Microsoft will make online Free supported by Ads and a Plus-type subscription
that will be so funny/
If that were the case the current subscription SHOULD convert into whatever their Plus-type thing would be. There is no way (at least I want to believe this) that Microsoft could simply tell you that the additional months of Live that you paid for would basically be forfeit with nothing to show for it.