PMS is the coolest software, genuinely amazing what can be done. However, it seems to be missing WebM support? AFAIK MEncoder can cope with WebM. Well, WebM wrapper is VP8 video with Vorbis audio right? So we already know audio should be no issue and it's just the VP8 video.
I had a look in /renderers/PS3.conf but this seems to state what a device can natively handle. So perhaps it's somewhere in the source that is not allowing WebM/VP8? I tried changing the extension to see if it was ignoring WebM simply based on my original extension (.webm) but it wasn't, so I have a feeling somewhere in the code it's not allowed.
I didn't want to checkout the SVN as I am no Java expert, so thought I would raise a feature request instead. I hope very soon Sony will release a firmware update with a full HTML 5 browser and native .webm support, until then if PMS can server my WebM I would be over the moon.