Kindle3 3Gマップ

www.amazon.com>Books>Kindle eBooks>Kindle Support>Kindle(Latest Generation)>Wireless Coverage Map>3G Coverage in your Area
で確認できます。日本は3Gでカバーされてます。
EDGE/GPRSもサポートしているようです。
現状、3G通信費は無料ですが、次のようなことが書かれています。

General. Amazon provides wireless connectivity free of charge to you for certain content shopping and downloading services on your Device. You may be charged a fee for wireless connectivity for your use of other wireless services on your Device, such as Web browsing and downloading of personal files, should you elect to use those services. We will maintain a list of current fees for such services in the Kindle Store. Amazon reserves the right to discontinue wireless connectivity at any time or to otherwise change the terms for wireless connectivity at any time, including, but not limited to (a) limiting the number and size of data files that may be transferred using wireless connectivity and (b) changing the amount and terms applicable for wireless connectivity charges.