There are three API standards I am aware off that are seeking the holy grail of homogeneous access to heterogeneous clouds.

The ones to keep an eye on are:

• DeltaCloud : An initiative from RedHat. This a a generic RESTFul service that mediates access to the Amazon EC2, RedHat Enterprise Virtaulization Manager (RHEVM) and Rackspace (though its not clear whether slicehost.com is supported. Its targetted at spinning up compute nodes so there doesn’t seem to be a storage, queueing or entity store API yet.
• LibCloud was originally developed by the guys at CloudKick but it has recently been promoted to an Apache incubator project. Its a Python library which is again focussed on compute server management. It supports a lot more providers including Amazon Ec2, Slicehost, Rackspace, Linode, VPS.net, RimuHosting, GoGrid, Terremark and VCloud (phew!).
• SimpleCloud is an mega corp initiative by the big boys (Microsoft, IBM, Zend) and is focussed on enabling cloud APIs in the PHP space. The website talks about blob storage queuing and structured storage (what SimpleCloud call document storage, which makes no sense to me). However the download they supply only supports the storage API at the moment. Surprisingly they have no support for managing compute instances. The APIs supported for blob storage right now are Microsoft Azure, Amazon, Nirvanix and local storage.