Yes, it is known that Polyphony check the boards. Exactly who does the checking and how frequently they do it is unknown to us. If any registered member of the forum is a Polyphony staff member, they have not made their presence known to anyone - not to the community in general, not to the moderators and not (as far as I know) to the administrators. To the best of my knowledge, no member of Polyphony Digital has ever directly posted here.
On that note, I would advise against using the forums to contact Polyphony. GT Planet is not affiliated or associated with them in any way. They do not come into threads looking for requests and ideas for future games (or updates); as far as I know, they only use the forums to gauge reactions to a game after it has been released, largely because the forums represent one of the biggest fanbases for the series. We are, in a manner of speaking, the pulse, and Polyphony occasionally keep a finger on us to see how things are going.
Any posts made attempting to contact Polyphony will be deleted on sight. We have no way of verifying whether Polyphony will even read them, much less whether or not they will act on them. With that in mind, these posts could reasonably classified as useless as they serve no purpose that can be confirmed, and as soon as one person attempts to contact Polyphony with requests for cars, circuits features and so on, everyone will. All that will do is clutter up the forums.
As such, I am going to close this thread before people start using it as their own direct hotline to Tokyo.