Ran across this twice, with two different books. Starting a paragraph with something like this
<p> <span class="i">
…ends up with the paragraph not rendering at all on some readers.
(Including, I believe, iPad.) This apparently flakes something out in the process of going from ePub to Kindle files! Keep an eye out for this one!
[Edited 6/23/11: It was a Kindle app on iPad, not the iPad’s eBook reader that was causing the problem.]