They've open-sourced management code from their Infrastructure-as-a-Service cloud provider (like Amazon EC2) and their blob store (like Amazon S3), meaning you can use this release to build a public/private cloud provider.
The IaaS provider code ("OpenStack Compute") is a preview and they say October is their release date.
As an open source cloud provider codebase it joins Eucalyptus, OpenNebula and a few others -- however (and since I've not used the rackspace cloud I don't know whether this is at all accurate) it should be much more stable since it's most of the code running a big cloud provider (whereas Eucalyptus and OpenNebula still tend to be rather flakey in my experience).
What's not entirely clear is how much they're holding back from the release - they say OpenStack Compute is "the same technology that underlies two of the largest and best ones out there". If they push features over to the open source project to keep parity with their own implementation then it may gain (or, in fact, may already have - I've not looked into the code much) features such as energy efficient host cluster management, shutting down VM hosts when they're not required.
There isn't really a major risk at the minute to doing this - the technology behind cloud providers is pretty simple, the cost of running a public cloud is buying and continually refreshing all the hardware and getting plenty of bandwidth and datacentre space to run workloads. Anyone with the money to spend on the hardware either probably doesn't mind spending some developer time building their own API (especially since if their customers use the API directly then having your own API is a good way to discourage customers from moving to another provider)