Oh man...so tempting to get a Kindle now despite having an iPad 2.
Nice thing is too, for the books that you purchase, it will automatically sync your last page read ... so you can start reading on the Kindle, switch over to iPad, go back to the Kindle ... all without losing your place and while maintaining your highlights and notes.
Maybe they'll loosen that restriction after the holidays when the free Prime trials from the initial round of Kindle Fire sales will have expired, potentially adding many new Prime subscribers.