There are several problems with this survey.
First, the text preceding the link says "Visitors". This makes it unclear to me (and I'm sure at least some others) whether or not it's meant only for people coming for the first time (as opposed to "regulars", for example). So if they're worried about bias like you suggested rdowns, right there I'm sure they've already got some people confused about whether or not they should be taking the survey.
Second, as others in the thread here have said, it says "survey for MacRumors". This is misleading, and I would guess intentionally so. I thought it was about my user experience on MacRumors, or something similar related to the site. If it said "survey from/by one of our sponsors" or "survey to help support MacRumors", people would know or at least suspect that the poll itself has nothing to do with MacRumors. Some people would be less likely to do it, which is probably why it isn't explained.
Third, there is no mention of who the survey is by, or who it is for, and what their privacy policies may be. What information if any will Survey Monkey collect or provide to MacRumors or the sponsor? The sponsor need not be named if it would bias the data, but there should be some indication that the data aren't being collected just for MacRumors' use.
There is an issue of trust here. The first question had me kind of suspicious, so I looked back on the MacRumors front page and this thread was listed on there. So I didn't continue the survey.
I thought since it was a survey on MacRumors, a site that I trust(ed), that I would be fine taking the poll. But now that I see that the purpose of the poll is hidden, and it makes me feel like that trust was broken.
MacRumors certainly has the right to get revenue, I just think they should make it clear when that's what they're doing, just like is done when threads are posted about sales from MacRumors' partners. It's a little annoying to have such threads, but I have absolutely no problem with it because I realize they have to make money, and they are very clear about it as they should be. I do have a problem with a link that appears to be intentionally vague, with no warning that you're giving up information for use by another entity.