I think the point of the forum is to ask other users about problems and look for solutions.It would be nice to think we are not just pissing into the wind.
I'm willing to bet they do have people scrubbing these forums. When I was running a MINI (car) site, we had two people from BMW on them. They would never say they were, but on several occasions we noticed the IP addresses that they were surfing from were registered to the BMW in Oxford. Once confronted they admitted it and asked us to keep it quite. These forums are a wealth of information for new ideas and to help resolve issues, why wouldn't you want to have your company monitor it?
MacRumors should do an IP scan to see if there's any from Apple (if there would be a way to identify them). I'm kidding, I don't think that will ever happen!
My gut reaction is that Apple would be naive to not browse around here, but they keep quiet. There's a few posters who seem to have had extensive knowledge of Apple's inner workings, but they've left...
Apple employees are under NDA, that is why they have to keep quiet. Several accounts have been guested (i.e. disabled) by users' request because they started working for Apple. I think part of the NDA is that they are not allowed to have an account in Apple related forums, or at least they are not allowed to talk anything there. Of course, this doesn't apply to all Apple employees, only the ones with access to secret data (i.e. Apple retail store staff might not have to sign NDA since they get no secret info). I know at least one member who used to be a tier 1 Apple technician and he had an MR account for the whole time and still uses it frequently (he said he didn't have to sign NDA).