I've been studying collaborative design early in my Ph.D. (contact me if you want a literature survey draft that I wrote about it a back in 2003).
Anyway, collaborative design applications (as in UML modelers) fall into three categories in terms of timing:
- Synchronous - Two people or more editing at same time
- Asynchronous - Check-in check-out model, a mess if multiple people edit at the same time.
- Hybrid (can share certain things in real time).
In addition, they fall into three categories in terms of metaphores:
- Desktop based - Essentially something like rationale rose with multiple user support
- Whiteboard based - Free canvas, not necessarily structured, sometimes has support for UML recognition. Usually a mess to manage multiple models.
So this gives you a 3x3 "design space" of tools, and there are research tools inside every one of them.
The problem is that in switching to collaborative work there are many usability issues that are difficult to address. For example, access control, synchronization, awareness, shared viewports, etc. There are some academic advances on these, but they're not necessarily in tools yet.
If this is the topic you're interested in, comment, and I'll post some of the tools I'm familiar with.