The change was one that many people missed during hand-on with the new Amazon tablets yesterday, but it was noticed by Elaine Fiolet over at Ubergizmo. Unfortunately, there is no word yet as to whether or not there will be an option to change the default search provider on the new Kindle Fires. Of course, even if you can't, it probably won't matter all that much.
There will be some that claim that Google is by far better than Bing, but we've never really seen that big a difference. Sometimes Google is better, sometimes Bing is. Either way, you're bound to get a lot of overlap, because the Internet is the same, regardless of how you search it.