Crossway has just made available an offline ESV Bible for iPhone. Compared to the other bible apps I have used, it has the best user interface. The chapter flows continually without having to click on a "next" button and wait. You'd just scroll and read on. There is also an option to add your own notes to the verses that come with links to their cross reference verses.
The only weaknesses are:
(1) there is no indication on the verse stating that you've added a note
(2) there is no back button after clicking on a cross reference verse
But all in all, I will be using this app during Sunday worship, though I might be still getting frowns from others thinking that I'm texting on my phone when I'm actually following the sermon with my ESV Bible.