Derek Lowe notes this effort by several drug makers to share data from failed clinical trials in Alzheimer's disease. The reason we do not have very good treatments for Alzheimer's is that it's a very tough nut to crack, and we're not even sure the conventional wisdom about the mechanisms (the amyloid plaque theory, for instance) are correct. The hope is that in sharing data from a whole string of failed clinical trials, someone will be able to find something that can move a cure–or at least an effective treatment–forward.
It should be appreciated that participating in this initiative is not easy. Desire to protect the privacy of research participants is embedded deeply within the clinical trial process, and if any of the sensitive personal-level data is to be made public, it has to be anonymized (and documented).
The data is also very expensive to collect, and the desire to protect it vigorously as a trade secret is very strong.
I think this effort is notable in light of the drive toward open data discussed by Tim Berners-Lee in his recent TED talk. This effort seems to be the first of several in difficult diseases such as Parkinson's. Stay tuned, because this will be something to watch closely.