kindle-feedback@amazon.com
However there are approximately 3 to 10 threads a day about this issue on the Amazon Kindle Forum, plenty more here and I'm pretty sure Amazon well knows the ePub/azw debate backwards and forwards. Jeff Bezos even gave an interview a couple of weeks ago where he talked about why Amazon is holding firm with their proprietary format. I found his tech explanations a little shaky but I did get the clear impression that Amazon really doesn't want to change their focus right now.
By all means send off an email to let Amazon know your opinion - no harm in that. From what I can tell, as of right now Amazon seems to be holding their major market share of eBooks just fine without ePub support. Until that changes I personally don't see them budging on this - it's probably a better business decision for Amazon to not give the #1 digital reading device the ability to buy all their books from places other than Amazon.com