The longer Amazon goes without releasing an instant video app for iPhone, the more likely it seems that an Amazon phone is imminent. If it were far away, it wouldn't make sense to hold off issuing. And there is no technical reason not to release (as there is an app for the iPad). So, my only explanation (I'm sure there are others) for no iPhone version is that Amazon wants to have as complete a differentiated feature list as possible.
I think (I could check but am too lazy) that Kindle books also followed the same pattern (Amazon proprietary device first, app version later).
UPDATE: I was clearly wrong.