homepage

AFAICT, there is currently no entry for WebM in IANA's registry (http://www.iana.org/assignments/media-types/index.html ).
A bug in WebM's tracker regarding its MIME type(s) was closed over a year ago as WONTFIX (http://code.google.com/p/webm/issues/detail?id=15 ).
Giampaolo has already pointed out the relevant part of the WebM spec. WebM itself has no RFCs (there are 2 relating to VP8, its video codec, but they don't have anything that's relevant here).
To independently corroborate the existence of the MIME type, we have the following:
Mozilla, Microsoft, and Opera all acknowledge the MIME type in question:
https://developer.mozilla.org/en-US/docs/Media_formats_supported_by_the_audio_and_video_elements
http://msdn.microsoft.com/en-us/library/ff975073%28v=VS.85%29.aspx
http://dev.opera.com/articles/view/opera-supports-webm-video/
The de-facto *nix MIME type database/package maintained by freedesktop.org includes it:
http://cgit.freedesktop.org/xdg/shared-mime-info/tree/NEWS?id=391ae76f6a2f3af93406ac31bc8735695f41d07d#n92