When I initially signed up for a small Rackspace Cloud server, I discovered the setup was 100% from the ground up. I chose to install Cent OS 6.0, but installation and licensing of cPanel/WHM was up to me - if I wanted to use it. I opted to do so.
I read around the internet and found a disturbing quote that I believe was taken out of context. It insinuated that cPanel/WHM was not compatible with Rackspace Cloud Servers.
I can tell you with absolute certainty, based on Rackspace's own response, and my own operational server, that is is 100% compatible. However, there are some restrictions, particularly if you have more than one server and are using their cload load blanacing. Also, if you wish to use cPanel's domain name management, you'll need to use your own server as its own nameserver (by registering it as a nameserver such as ns1.example.com). You must usually get your host to do this for you, they'll know what you mean.
It isn't technically the best nameserver setup, but if your server is down, resolving it doesn't really matter much anyway ;p.
At this point you are in full control of your virtual server, nameserver included. All of cPanel/WHM is too.
Rackspace Cloud does have some client services it runs on its boxes, to keep the monitored, but I've seen no problems, and was told there was no significant problems in a response to a support request. In fact, I was told many of their customers use cPanel with a Rackspace Cloud Server.
So, to set the record straight on this ... you CAN use it, no complications, but you do have to pay the cPanel licensing fee, and you should be aware of the context you are running it in, and that certain operations could cause them (Rackspace) to make modifications to system or service config files as well. I've not had a single issue though, and I've done quite a bit of toying around this first day or two.