Android should "just work" but there are some regressions since I last tested it, from what I hear. Hopefully we can get these examined and fixed soon.
There are some possible things we can do launching files in the VLC app for iOS; some clever JavaScript could make this experience smoother. I plan to do a little more research when time permits (after Architecture Summit this week).
There are some vague ideas jumping around for linking a proper WebM video player into our iOS app, but we haven't committed to such a thing yet.
-- brion