Even after getting sideloading to work, are you guys getting bookmarks, furthest page read, etc. also synchronized between iOS Kindle and other Kindles (other iOS devices, physical Kindles, etc.)? From the post above me, it seems very problematic (one way only, having to delete and reload books, etc.)
When I originally tried this on my iPad (before version 3.0), I was able to sideload with the help of members' instructions here. But I was never able to get furthest page read synchronized. So for my personal needs (I have an iPad and an iPhone), I ended up just using iBooks because furthest page read is always in sync between both, including sideloaded content.