Rackspace cloud servers are basically what slicehost offers. They don't have cpu bursting. Instead each slice on the host is guaranteed a minimum amount of cpu and any unused capacity is available for other slices to use.
So if you have four slices on a host and 3 slices only use 10% each of total cpu capacity. Each slice is guaranteed 25% but they aren't using all of their guaranteed cpu. Your slice can then consume the remaining 70% of the cpu on the host. (Minus virtualization overhead)
Unlike bursting, you don't get throttled until other slices need cpu. Your slice can happy keep consuming 70% until the other slices start asking for their share.
I know Cloud Servers originally started as a direct copy of slicehost with hourly rates and end of the month billing instead of monthly prepaid billing (prorated per day). A few things have changed since then and I don't know if slicehost and rackspace are still offering the same system any more.
I do know that the vm environments are the same. I'm just not sure about some of the other bits.
If you go by the look & feel of their web interface then it certainly still feels like a bolt-on. I also remember various of their server ips to have PTRs to slicehost names, but that may have changed since I looked (a couple months ago).
So if you have four slices on a host and 3 slices only use 10% each of total cpu capacity. Each slice is guaranteed 25% but they aren't using all of their guaranteed cpu. Your slice can then consume the remaining 70% of the cpu on the host. (Minus virtualization overhead)
Unlike bursting, you don't get throttled until other slices need cpu. Your slice can happy keep consuming 70% until the other slices start asking for their share.