One big wiki is one approach for bringing together all manner of related subjects - e.g. sustainability with green living, sustainable design, open design, international development, appropriate technology, etc.
This has the advantage of tremendous synergy, less work doing the same things (both tech and content related) in multiple places, and more connections.
Potential drawbacks may include a concern about loss of group identity. Social features could alleviate this, particularly those that enable groups to set themselves up in Appropedia with email and/or easily tracked discussions.
See also
- Federated wikis (speculative)