I'm Rémy from the SublimeVideo team, thanks for this plugin, I think it helps many people to start using SublimeVideo in their WordPress blogs. However, we think it has some limitations that's why we are in the process of building an official plugin (with a "video embedding helper" etc.).
Just to clarify the question above (even if it's a bit old now):
Using .m4v AND .mp4 is indeed unnecessary, as explained in our documentation "we recommend linking directly to a MP4 source (href attribute) to enable compatibility with mobile devices where the lightbox effect is currently not supported.". So the real point is to put a link to a mobile-compatible source (i.e. a H.264 file, it can be .mp4, .m4v or even .mov). In that sense the plugin is a bit misleading since it forces to set a m4v source where you could put virtually any source you want.
Also, I got some feedback when a user wanted to use the "m4v" attribute without the "zoom" class, in that case it shouldn't set up the lightbox effect.