My problem is not WordPress, with Kubrick theme (default) the video looks fine (with plugin or not). I use Viper's Video Quicktags and the only change is that show a thumbnail instead of video.
The problem i have is with my theme (Langit) and others like o2, Minyx, etc... I think that is a problem of some code or tag.
I've decided to change my theme for another one, because i don't know whats wrong xD! I don't want to change because i expend much time modding... but I don't see another solution...
MoTStudio thanks for try to help ^^
Edit: Yeah!! xD! i've found the solution (in the forums of Viper's Video Quicktags):
I knew something was wrong with the theme code =P