Basically my understanding is, if you send something to
yourname@free.kindle.com it will always be free and always delivered over wifi only.
If you send it to
yourname@kindle.com it will be free over wifi but charged over 3g but it will always be delivered to you as long as your device is in range of a cell tower or wifi.
If you set your spending limit in Manage Your Kindle on Amazon to 0.00 then only the free.kindle.com is used and documents are delivered only over wifi.
If you have just the Kindle 3 wifi, then it doesn't matter which address you use, it will always be free.
Otherwise browsing over 3G and downloading from Amazon is free.
Hope this helps(?)
Cheers,
Andrew
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