I'm hard-pressed to think of a reason why this thread should be left open. Firstly, there's a whole lot of AUP-related issues going on with an influx of spammy posts. Secondly, it's well-established that online petitions do nothing. I have only ever heard of one that actually succeeded, and that was put together by fans of CBS' Jericho to stop the show from being cancelled. Even then, it only lasted half a season before it got cancelled a second time. If an online petition cannot save Firefly from cancellation, this one probably don't work. Besides, petition threads are a pet hate of mine, and I have half a mind to close it right now on that basis alone. However, I am (usually) a level-headed individual, so I'm not going to act rashly.
Of more pressing concern to me is the way this appears to be addressed to Polyphony Digital. This is a big no-no. We are a fansite, and out involvement with Polyphony begins and ends there. We are not your hotline to Kazunori Yamauchi. While it is known that members of the development team have visited the forums on occasion, no-one has ever (legitimately) come forward and made themselves known. Thus, we have no way of knowing who they are, when they come online and what they view, much less whether they make good on it. Therefore, any post or thread addressed to Polyphony may be considered useless, because it serves no purpose. I'm in the habit of closing them on sight.
And, on a note related to the topic, it's not simply a case of Polyphony deciding that they want to include Spa in the game and going ahead and modelling it. They need a licence, and since the circuit already appears in F1 2010 and several others, the owners of the circuit may not be enthusiastic about having it included in GT5 as well.