Sun and Microsoft's recently announced Web Single Sign On Interoperability Profile defines both how the Web Single Sign On Metadata Exchange Protocol can be used by providers to determine what SSO protocol suites another uses (e.g. Liberty ID-FF 1.2 or something else) and some slim profiles of those other protocol suites.
The profile specifies that, to be compliant, you MUST only use the Liberty ID-FF Browser POST Profile - the other profiles (e.g. artifact based) are excluded. WS-Federation is similarly constrained (although WS-Federation doesn't have anything comparable to an artifactt so there is no need to exclude it).
Although not stated in the document, the presumed goal of constraining both ID-FF and WS-Federation in this manner is to align the two of them into the same message exchange pattern (MEP), specifically one in which service provider and identity provider communicate with each other 'through' the browser using HTML form posts. I guess the thought is that by converging on this MEP, either provider can easily swap in or out either protocol, differing as they are only by XML syntax.
This seems a new twist on enabling interopability through constraint as practised by WS-I.org (perhaps why this work was not done within that organization?). In WS-I, groups of complementary standards are constrained as a group so that the end result is a consistent and interoperable combination.
The 'interoperability profile' of Sun and Microsoft seems a different beast. In it, two equivalent SSO suites (varying along the standards body ratification scale) are individually constrained to bring each into alignment with a common pattern. This is a different type of interopability, in no sense could it be said that ID-FF and WS-Federation 'work together' as do SOAP and WSDL in the WS-I Basic profile. Rather, by aligning with this common pattern, what is being enabled is the ability for providers to easily switch (even at run time) between both suites.
As 'meta' seems to be enjoying a resurgence in popularity lately, let's call this type of interoperability 'meta-interoperability'.
Maybe the 'interoperability profile' will even eventually be submitted to MWS-I.org?