Macbook Air
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I dont think this is going to happen, because everyone knows its the Air's weakness not having an interchangable battery so it will become the Pro's weakness too, also Apple like to keep their product ranges similar, so if the 17" is getting an internal battery, i would think the 15" would be too.
No, not everyone..... my wife has an Air, and uses it exclusively when traveling. Not once has she ever thought - "Gee, wish I had an interchangeable battery." When its time to get a new battery, we'll take it to a store (Apple or authorized) and have them change it.
I have a 15" MBP. Except for the first day, when I was checking it out, I haven't ever taken the battery out.
I do, however, constantly locate and sit near power plugs at airports and on ferries in order to plug in. To have notebook that would last for 6 or 8 hours would be wonderful. I don't want to carry a 2nd battery, I like to travel light. So, if true, I say bring it on!