Version 0.6 of the External Video for Everybody WordPress plugin allows you to designate (on the options page or within a shortcode) whether or not you wish to include a poster attribute in your HTML5 video tag. Dropping the poster follows Video for Everybody 0.4 in order enable playback on iPads. Leaving posters as an option allows each user to decide globally or on a video by video basis. You might even turn posters off but continue to create and upload them with the idea that you can turn them on when iPads can handle them. Or you can just start making your posters into the first frame of the movie.
Additionally, version 0.6 offers experimental support for video URL's that require a query string after the file name. Amazon S3 has been pointed out as a case in point, but I do not use the service, and so I cannot test whether my accommodation actually addresses the problem.