We've been doing some testing with Apple's production release of iOS8, naturally focusing on the mail client, and one of the main findings so far has been a lack of support for the tag.

Testing our recent studio moving email loads the poster image, but the play button doesn't seem to function properly.

We've also done some initial testing around different file formats and file names, but we've not been able to get past the bug yet.

It does at least feel like this could be the issue though, as it does seem to attempt to load the video.iOS, and Apple platforms in general, have a good level of support for HTML5 (as far as email goes), so it remains to be seen whether this will be fixed in a future release.