They want to fork the project and build a new UI? Cool. Go do it. If people like their changes they'll start using their version of Adium, and other people will contribute to their project.
It's not just an idea. It's a considered UI for the community to critique and possibly implement. An open source project thrives on multiple people to help out. If this mockup was so simple and obvious, it would have existed already.
To follow the analogy, one could use the Cowboys stadium as a precedent and then adapt it to the climate of Minneapolis.
This isn't a discussion about the codebase because the code can't solve the questions being discussed. There isn't anything in the concept that is out of the realm of feasibility. Critiques happen at all phase of design and development, and you seem like someone who's uncomfortable with ideas that aren't finalized.
Anybody can make something pretty in Photoshop. If he was serious about getting people to implement his mockup he should have spent a few days writing a quick and dirty implementation, with real code that could be improved upon later.
What's interesting is the interface, which is why we're discussing it. The fact that you can't run an application doesn't make the concept any more or less interesting to discuss and critique. The issues being discussed aren't solved with code.
You know that there's a profession called UI Designer?
If I had the time and knowledge of the Adium code base I'd be thrilled to collaborate with these two talented individuals. Hopefully there are developers out there with open enough minds, time, and passion to work with these two.