I recently took a look at Delta Cloud - a good start at providing some
ability to switch "between clouds" or at least work across them
(including "clouds" based on virtualisation managers like RHEV-M from
Red Hat and VMWare).

Rackspace seems pretty sweet, has a nice RESTful style API to manage
stuff, so it was easy to write a driver for deltacloud.org (which I
did):

Get the good stuff here: (should be upstream shortly)
http://github.com/michaelneale/driver-rackspace/

The deltacloud framework itself is a RESTful rails app (and the
"drivers" are written in ruby).