A person leading a usability seminar once cited research that the number of people in a brainstorming group mattered not as much as the diversity in the group. People with different backgrounds tend to have divergent ideas and often they can help “fix” incomplete or inadequate ideas of other people with their own specialized knowledge approach. Not sure that writing ideas is effective either. People filled with ideas often abbreviate their ideas on paper, and less verbal people will undermine their own ideas by writing them down. There needs to be some presentation to the group about these ideas. In my work with aptitude testing, I saw wide discrepencies among individuals about idea fluency. Unless the research factored that out, I tend to think that a lot depends on the individuals being sampled. Group brainstorming sessions can be good for making sure that ideas are focused. Also more importantly, public airing of ideas ensures that people are not simply repeating the ideas of others.
The most interesting finding from the article is about the necessity for brainstorming multiple times (i.e, having one session, taking a break and then doing it again). This makes a lot of sense.
Another brainstorming idea (not mentioned because it is perhaps a bit too obvious) is web discussion boards. The power of slashdot provides a lot of different insights into a single topic. It probably could do the same if you have enough eyeballs.