I am not aware of multimedia specific patent pool. However, the recent Google Oracle Java case is good counter example. Both Google and Oracle have been part of OIN, but that did not stop Oracle from suing Google.
The WebM Community Cross-License Initiative launches
Posted Apr 27, 2011 15:53 UTC (Wed) by JoeBuck (subscriber, #2330)
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That's because OIN has a very narrow definition, preventing people from suing each other over the Linux kernel and a specified set of common packages (including gcc). Java, however, isn't on the OIN list, which is why Oracle could sue Google.