Thanks for the link.
> Notice that permission is only given for implementations that comply with the specification.
That's typical - otherwise, you could implement some almost arbitrary protocol but say it was this one (and use a small amount of it) and claim that you were shielded from Skype's patents. The patent license is just for this particular codec.
Of course there are issues with this, it's been debated a lot regarding the Java and C# licenses, which also have similar clauses for similar reasons. The motivation for them is reasonable, but the details can be tricky.
> (And if Skype ever infringes any of your patents, you can't sue them without losing the rights to Opus.)
Personally I think that's fine. If you want to engage in a full-on patent war, you should not benefit from a patent license like this.
> Whether or not you agree with Skype's moral right to its patents, this is clearly a limitation of developers' freedom. Imagine if HTTP were covered by this kind of patent: anyone could implement a client or server, but developing a backward-incompatible extension like SPDY would be off-limits. If you were to take the license terms literally, even releasing a buggy implementation that doesn't precisely follow all of the requirements of the spec would make you liable.
If a single bug makes you liable, obviously it is unfair, etc, but that's exactly where the details of the license come into play - I am not a lawyer, so I have no idea if the terms here are reasonable or not.