I think anyone who is in the technology sector in any way should be worried about Google. Google puts out excellent products and seems to be loved by everyone everywhere. They have the air to them that they can do no wrong, and people will choose them over Amazon if need be.
Amazon has it's thing and does it well, and Google has it's thing and does it well. I don't see where the two cross into each other's territories. I love Google for what it is and use no other search engine, at least initially. And whenever I purchase something I check Amazon before anything else. I see myself continuing to use both for different purposes.
No, I don't think that Amazon should be concerned about Google. While Google is on the rise, I think that Amazon is as well. For me personally, I buy practically everything on Amazon. I also own a Kindle Fire HD. Many people I know can say the same. Amazon is huge and does not need to worry.
Google is not a threat to Amazon right now and will not be any time soon. Amazon is more popular and better than it has ever been. I see and hear about new people signing up for Amazon Prime and Video every week. Google has a long way to go.