Thanks Theyetus.
As I understand it, the answer is that I must be plat at the time the person gifting me the award reservation adds me to the reservation. And, if I further understand things, I must be added to the reservation at the time the award is issued. (Terms 3.1.c.)
So, what this effectively means is that someone in my situation cannot get plat benefits for a gifted award stay. For example, my brother-in-law makes the award reservation for me today for a stay in December, but I don't make plat until September. I'm out of luck, so far as plat benefits are concerned when I stay in December, even though I will be plat at the time.
That to me seems like a bit of a stretch under the terms. So long as I provide a plat number at check in, it would seem under the terms that I should be eligible for plat benefits. Starwood could, I suppose, take the position that an award gifted by one member to another is not an "Eligible Stay" at all -- even after October 1, 2011, because the Member did not actually redeem the award with points out of his own account. It think that would stretch the terms, and it doesn't seem consistent with the practice, but it would not be a totally indefensible position to take. But, barring that, it's hard to figure why a plat who is plat at the time of the stay wouldn't be entitle to plat benefits. All that said, (1) it's too small an issue to be too worried about, and (2) I'm being given the stay for free, so I'm not really complaining about any benefits.
One potential way around the problem is if a member could make an award reservation and then add his giftee's name at a later date. So, for example, my BIL could make the award reservation now to ensure availability, but then wait until September when I make plat to add my name. It seems like this is not permitted under the terms, but perhaps this is a rule honored in the breach.