This is an OT reply to an OT, so skip over if not interested.

While I have only been employed for a few weeks now, I don't have much experience with end users, but I can see why some people might get paranoid about their privacy and anonymity (especially after knowing about all the sites that collect user information). To help with this, you might want to add a FAQ page with questions and answers about these issues (and mentioning that anonymity could be compromised in the old pencil and paper surveys). You might also want to add pointers to "Anonymizing Proxies" that people can use when filling the surveys (not that you can actually trust people running these proxies). Getting a third party (the CS department for example) to set up a proxy that people (from campus) can use could very well be a solution.

Hope this helps,,,