Originally Posted by rogerinnyc
On an iPhone, the folder is var/mobile/applications/kindle/ebooks. Of course, to get a file into your iphone, it's a whole lot easier if you're jailbroken (i don't know how to do it if you're not). Hope that helps.
FWIW, on my iPhone with iOS4 the path is:
But yes, just dropping a DRM-free .mobi file in works well.