Lets not get deceived by the shiny brochures and the hot marketing air offered by propriety system. Open Source systems have a lot to offer.
A couple days ago, Google opened up it's social network platform Google+. I've already seen modules/plugins for WordPress and Drupal (both open source software). Quality of the plugins is not yet great (already discovered some bugs, that I will need to report), but the software is running. I'm feeling bad for those poor people that are on a propriety system like Sharepoint. Those poor users will have to wait 1) forever and will 2) have to probably pay extra and that's why nobody wants the feature.
Top Management Does not acknowledge Open Source
Problem is: In the heads of the top management, open source software is still not mature and inferior to propriety software. Just compare Sharepoint and Drupal. There might be usecases where Sharepoint is superior, but then there's also many usecases where clearly Drupal is better suited (yes also in the industry). In the heads of the top management they know Microsoft, they know it will be around in a couple years, they have nice brochures and they have a special enterprise solution (that probably just has a higher price).
The Role of the Project Manager
Lets not make the same mistakes. Lets not get deceived by shiny brochure and hot marketing air. As project manager we might not be able to choose the system but we can at least try to influence it. I have now seen projects from both sides. I have worked for many years with Drupal. Not everything is gold! I have worked with Sharepoint from a project management position and believe me: there's much room for improvements! Lets give Open Source Software at least a chance.
The success of an using open source software (lets assume it's a well established OSS) in the enterprise depends on the quality of the team/vendor supporting the system and their knowledge. In the wrong hands this can go totally wrong, but if you have a good team and lots of knowledge you can pimp that software in many ways to do things beyond your wildest imagination!