I wonder whether there is already a "standard" for tasks like specifying the language something is written in. At the time of writing this, I use tags like "lang:en" to mark something as (here) English. Following the machine tag scheme, it should be read as "lang=en" without a namespace part (similar to the "lang" HTML attribute). The namespace would have to be a reserved global identifier along the lines of "meta".
Are machine tags useful at all? Or is there a better way or microformat to implement their functionality? I have no idea, but this is all quite fascinating.
On a lighter note: Merry Christmas and happy holidays everyone!
Update: I guess it would make sense to set the current domain name (or perhaps sub-domain/URL in case of multiuser sites?) as namespace if an explicit namespace has been omitted. Following this approach, it might be a good idea to associate every namespace with an URL. This would allow to link machine tags to web service APIs.