Are you serving the video with the correct MIME type? The excellent Video On The Web article by Dive Into HTML 5 covers everything you need to know about serving Video. Way down the bottom of the article (past all the encoding help) covers issues with iPhones and iPads as well as the need for the correct MIME type. It’s well worth the full read.
EDIT
To work with iOS the Accept-Ranges: bytes
HTTP response header must be included, see: Safari Web Content Guide – Configuring Your Server