As a practical matter, all of the existing interface systems (Zope, PyProtocols, and even the defunct Twisted implementation) treat interface inheritance as guaranteeing substitutability for the base interface, and do so transitively.
An additional data point, by the way: the Eclipse Java IDE has an adaptation system that works very much like PEP 246 does, and it appears that in a future release they intend to support automatic adapter transitivity, so as to avoid requiring each provider of an interface to "provide O(n^2) adapters when writing the nth version of an interface." IOW, their current release is transitive only for interface inheritance ala Zope or Twisted; their future release will be transitive for adapter chains ala PyProtocols.
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