Forrester Research published two reports on Drupal. A 6-page report on Drupal itself, and a 15-page report that compares blogging platforms (including Drupal). I don't have access to the reports but here is the executive summary that was published on their website:

Last February, Charlene Li, a principal analyst at Forrester, invited me to participate in the screener and information gathering process but I was too busy at that time. It was around the time that the Open Source CMS Summit in Vancouver took place. Fortunately, Bryght was able to answer their questionnaires.

More information is available at Charlene Li's weblog where she wrote about Forrester’s blogging platforms evaluation. The figure below is taken from Charlene's blog:

Forrester blogging platform
Source: Charlene Li's weblog. © Forrester Research.