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I can understand peoples view if they are looking at it from a .com, .net, .org point of view.
This is different from just releasing a new top level domain.
The difference is that to use a .mobi address you will have to abide by a style guideline. This will ensure that anyone people viewing it on a mobile phone will see the site correctly.
Whether this works in practice remains to be seen :)
Whether .mobi is implicit or not, users will know that all .mobi sites will work well with their phones.
If the browsers on phones do default to .mobi (as browsers on desktops sometimes default to .com) this will give .mobi instant typeins as well.