I really, REALLY like this idea. It would work just like the Ross-Tech wiki -
http://wiki.ross-tech.com, and as others have said, limit edit permissions to reliable, trusted members.
As a really valid example, it would seriously help dispel the myth, and associated crap around these unfounded claims that the Edition 30 engine is just a 'detuned S3 engine' when I know for a fact it aint. I have a fairly detailed spreadsheet of all the component part numbers from all flavours of the 2.0 TFSI engine, and I reckon that a 'Golf GTI wiki' would be a really good place to host this kind of information.
I'd glady help out with editing and adding stuff - providing you think I'm knowledgeable and 'trustworthy'