Standards an control...
Standards are, indeed, important to avoid lockin. I guess if one vendor starts becoming dominant then otheres will come along and reverse engineer (or rather, re-implement) the APIs to produce their own version. However, that begs a question - I assume that cloud providers have IPRs over their APIs and might be able to block such competition using legal means.
There is a further issue. Even assuming that there are no legal matters then just how compatible with environments be that apparently implement the same APIs. Anybody who has had the job of delivering non-trivial applications onto environments offering the same programming environment will have stories of incompatibilities and the need to do full testing on each. Even enviornments which were meant to be fully abstracted suffered from this - it wasn't for nothing that Java's original byline; write once run everywhere became known as write once, test everywhere. These cloud environments, with their complex eco-systems implemented using a range of underlying platform hardware and storage are going to be no different.
Cloud compatbility across vendors will undoubtedly be a good thing, but I'd invite anybody to look ate the issues over moving a complex application between different J2EE environments to see what the issues might be .There is a long way to go on this one before many corporations are going to entrust their mission critical systems. Of course there is a market for hosting those the messy little systems which any large company appears to accumulate, but even those have a way of infiltrating their way into important business processes. Some companies without strong procurement and standards imposition will undoubtedly find bits of their business dependent on more than one of these cloud suppliers as departmental managers get quick-and-dirty applications implemented on these environments. It will be the modern replacement for all those various apps which reside on aging PCs and server scattered around the business performing important day to day roles but which nobody has any central control. This time they'll just be vritual bits of processing and data scattered all over the Internet.