Sorry, I see this message very late: What intrigues me in this educational area is the product D3E from British Open University: http://d3e.sourceforge.net
You can place the object of discussion - could be a proposal of one of the students - on the left and then have a threaded discussion on the right. A kind of one to many writing relation.
The most sophisticated version is D3E: you use a tool to divide your text on the left in pieces (or paragraphs) and then you can connect discussion-start-entries to the header of each paragraph in the thread on the right.
There is an online journal already using this concept since 1996!!
The other version is Ubiquitous-d3e: you only point to the website with your resource and it will show-up on the left, discussion can start right-away on the right, this time without linking: for me this is one of the educational modules I wish for Moodle. (Of course after, the cohort is finished)
You can test the idea right on the spot on this website: http://ud3e.open.ac.uk
The other line I follow is that of the Wiki: I prefer the small, wonderful, standalone Swiki. (http://minnow.cc.gatech.edu/swiki) and I use it as a plugin for my Moodles until Moodle comes up with an integrated alternative. (which I would prefer)
The most interesting part of the Swiki is the possibilty to create on the fly fill-in-templates for the new pages of your Swiki, a way to structure student-activities with ques..
The reasons I choose Swiki was the easy way you could attach files and inline pictures to pages, create hyperlinks to new pages and can reshuffle the organisation of these pages when insight grows.. (the wiki-easy-edit-interface, now growing in Moodle, is only very short an advantage: very soon people are motivated to have more control over the visiualisation of their content on their pages and take voluntarily a manual)
Your Tikiwiki is also interesting: They try to create a kind of workflows inside the collab-activity in the next release..
The difference with Moodle is that the coaching and guidance in Swiki and Tiki is more outside the system, the focus is on the huge product students create together...
For courses I prefer Moodle, for collab-projects the other two could facilitate
There are more sophisticated collab-webtools, for example for creating concept-maps together, but that is to abstract for starters. Even Tiki or Swiki could suffer from that, so starting with a php-tool for collab writing in a book methaphor with built-in roles for students as chapter editors could be a nice start: http://craftysyntax.com/myscrapbook/ after that you could give them Swiki to structurise ther own research-project.