Oooh. This is just begging for a privacy firestorm when someone de-anonymizes the data, which I'm guessing won't be super hard given the kind of medical features they'd need to provide to make this task useful.
I'm not sure about that- unlike social network data, there isn't a publicly availible dataset containing the names of all the people in here which could be used to de-anonymize this data.
You're perhaps right. Of course, these people are on things like Facebook, Twitter and blogs and if you can narrow down age, gender, location and medical condition, you might be able to correlate with public posts.
You can also look for specific people in there if you have certain kinds of prior information about them: For example, Aravind Narayanan was able to de-anonymize some part of the Netflix set [http://arxiv.org/abs/cs/0610105]. Maybe that won't translate to this data.
