The Amazon Web Services Blog is linking to a Creative Commons license as an example of how "generous licensing" allowed IE7 to use A9's OpenSearch Description format for its search plugins, but the licensing only applies to the OpenSearch specifications (not to the use of OpenSearch, otherwise IE7 would have to be Creative Commons-licensed too).
The A9 Developer Blog also talks about how Microsoft encouraged OpenSearch to support multiple response types - search responses can now use "any" XML-based syndication format - which brings OpenSearch closer to SRU (and also picks up a lot of the complexity for which SRU has been criticised, while adding extra ambiguity for good measure).
The OpenSearch Description format seems like a good format for search plugins: Firefox should probably switch to using these too. I don't particularly like the capitalisation, but at least they're valid XML (which Firefox/Mycroft search plugins can be too, of course) and well specified.