I experienced the same issue with Kindle app on my iPhone 2G and 3G. Both are registered in Amazon, and I can buy books (using Gift Certificate loophole as I'm not in US), and I tried to email
myiphonename@kindle.com and
myiphonename@free.kindle.com. Nada. THe mail were sent, but no reply at all.
On the Kindle app hang opening ebook, I experienced it with the free ebook Resonance by Chris Dolley. Upon checking the header, it was indeed the TEXtREAd header that is causing this. I run mobi2mobi gui and get a new file with MOBI as header. All good now as I can just iphonebrowser-copy the new .mobi to the eBooks directory of the Kindle app.
For the metadata, using mobipocket creator, we can make sure the metadata is the same with the original file, if needed be, up to the ISBN entry.
As someone here mentioned in earlier post, I learned something new today.