[lr]

For the IdM side of things, there is SCIM:

http://www.simplecloud.info/

It started as Simple Cloud Identity Management, but once it got to IETF, it was changed to "System for Cross-domain Identity Management". This initiative has been backed by Google, Ping Identity, Salesforce, Cisco, SailPoint, UnboundID, and others. You can see some of these names on one of the RFCs:

http://tools.ietf.org/html/draft-ietf-scim-core-schema-00

This API is just for identity management, and it has taken years to get to this point. I think it would take many more to get to a common API for everything else.

[mehdim]

This is not always the "standard" which wins. You may have also de-facto standards like MS-DOS in 1980' which was an "shell to rule them all" and which has made Microsoft a multi-billionar company and has been more used than UNIX which was better and open!

Either is the standard, either it the the most adapted product/company (not necesseraly technologically) which wins the pot. This is technology AND business model, user community, strategic partnerships which will make an API to rule them all a defacto standard which will lead the programmable web. But you're right that IETF standards for APIs may be so long that I consider it is not possible untill decades !(or a leader which will open and free its winning standart to all only)