Auto scale is, in my opinion, one of the core features that a potential customer must evaluate in order to choose a cloud provider.
In short, auto scale is the feature that allows virtual servers to obtain new resources on demand without the need of a reboot.
AWS has implemented this technology in 2009, Google Compute and Microsoft Azure offer auto scale too even if its introduction was not exactly flawless.
Rackspace announced yesterday the availability of auto scale technology for its public cloud offering: Rackspace Auto Scale tool is driven by scheduled-based and event-based policies in order to cover most of the common use cases.
The details are available in this blog post.