John M. Willis has put together a matrix of the major players in the cloud right now including some of the companies that enable and others that are just offerings.
It's an interesting exercise to figure out how all these parts fit together and considering that the SaaS providers don't have a reason to disclose what's going on behind the scenes I bet there is a ton of other software that is not represented here.
Just knowing what I know about Mule's adoption in SaaS companies I can tell you that we are enabling several businesses already. It would also be interesting to know how many of these cloud companies are using open source--especially MySQL and Spring as part of their environments
Disclosure: I work for MuleSource, an open source software vendor.