Like has been said, we have seen Devs post in these forums on occasion, and there is the "Meet the Devs" thread, as well. They're probably not here nearly as often as some of us are (since they're, you know, kind of busy making a videogame and all that...
) And I wouldn't be overly surprised if there were some form of guideline dealing with when and how someone on the team posts on here. (Simply because every little thing a Dev ever post is, of course, going to get extra attention, and will be picked over with a fine-toothed comb.)
But also, like SystemShock said - by the time a forum comes up, there's already going to have been a significant amount of pre-production finished - at the very least. And it's also worth noting that I seriously doubt that one thing any game developer is ever going to be lacking in would be groovy ideas to add to a videogame.
Personally, I think there's always a chance that my views might contribute to a certain "gestalt" when the company wants to get an overview of how people feel about particular things; but really, the "point" of these forums is more about a community of fans than a breeding ground for ideas that might actually work their way into the game. We might all think we have some awesome ideas to contribute, but let's not forget these guys do this for a living. I kind of assume that in the unlikely event that any of my suggestions coincidentally get implemented into the game, that there's also a significant possibility that it was something someone over there had already thought of, anyway.
There was an interview a year or so ago with Todd Howard and Emil Pagliarulo where I seem to recall them saying that in a forum it's less about specific features that forumites are interested in, but seeing through all of that to get at the "core" of what they really want.