Picksit: To better understand the reasoning behind the introduction of the .mobi extension just ask yourself...what's the reason for the other varieties of extensions: .com, .org, .net etc.?
The .mobi extention has nothing to do w/ keywords directly. That's the WordRegistry.
IMO, the .mobi extension would allow those keywords though to be able to have more than one "response" from the WordRegistry servers...either a standard PC protocol configuration or WAP website would be "called up". I'm sure the "switch" will be able to determine if the Word "request" came from a landline internet connection or from a mobile internet connection and direct accordingly.
It also provides a structure to create better standardization methods.