I was talking the the Rackspace dudes at PyCon (I'm a Slicehost customer since before they were bought) and apparently the same backend infrastructure is cheaper if you buy it on a 'Rackspace cloud' branded plan rather than a 'Slicehost' plan.
That's not necessarily true, because the RS Cloud rates don't include any bandwidth while the Slicehost rates do. Depending on your usage, Slicehost can be cheaper.