I left the following comment at CloudAve yesterday, on a post titled Open Federated Clouds And Sun's Cloud Announcement.
Interesting. Looks to me it all depends on how you look at different clouds - as infrastructure providers or as software platforms.
The former case is roughly similar to buying Internet connectivity for your office from 2 different ISPs for redundancy.
The latter case, however, is roughly similar to a process of selecting platform for a project - say between Weblogic and JBoss. For a new project, a single platform is usually selected - I don’t think there are many cases when an app is built on top of both for better resiliency or to increase capacity (even though I admit that it’s not impossible).
In both cases, products are very similar or nearly identical to a certain extent, but the way you look at them makes you select 2 in one case and only 1 in another.
Right now, I think choosing a cloud is akin to selecting a software platform. So one will choose only one. However, the future may very well change this trend like you said, especially as interop gets better and each cloud gets its strengths and weaknesses better defined.