The basic idea of this session seems to be how to get over the difficulties of Drupal shops working together. The general consensus in the room is that most Drupal shops are 10-15 members. Working with other groups is not just about collaborating on a client project, but also collaborating with groups that are responsible for published modules.
One of the primary blocks for shops working together is communication between collaborators. By keeping the communication channels open, things like duplicated module work can be avoided. Keeping the plans open in spite of competitive differences that might otherwise prevent collaboration.
This can be accomplished by blogging what you're doing to an internal back-channel. Do research before you start coding, and document what you all need to do so that the scope of work for each person is clear.
Also, if you're working on a module that others might use, be sure that you publish a path to completion or a list of things you might do in the future so that others who use your work know the direction you're heading in.
Using the correct tools, like blogs, offer unexpected benefits to collaborative efforts. Traditional tools like CVS can also obviously allow you to collaborate better with other groups.
Maintaining modules that you develop in congress to prevent them from languishing in client projects is one way to benefit from collaborating with other Drupal shops. By maintaining the modules together, you could re-use those modules later in future projects.