If you are going to be providing shared and reseller hosting on Unix and Windows whereby you want to manage the physical machines involved as a cluster, please consider H-Sphere from http://www.psoft.net/
H-Sphere provides complete hosting automation which includes a control panel for the hosting provider, the reseller, and the end user customers of both.
H-Sphere allows for multi-domain hosting across one to many servers which can be running various operating systems -- CentOS, FreeBSD, RedHat Enterprise, Trustix, Windows 200x -- and located at various locations. Yes, your cluster can be spread apart in various data centers around the world (though mysql servers should be in the same network segment as the web servers to keep latency low).
H-Sphere allows for instant sign ups with fraud prevention through signup guard; and there are add on's to help with fraud prevention from http://www.hspheretemplates.com/
H-Sphere licenses recycle automatically so as accounts are deleting, the licenses are automatically freed up to be re-assigned automatically later.
While H-Sphere has an extreme learning curve compared to systems which are just another control panel, once you get past the learning curve hump, the rest is down hill (where as most just another control panel systems there is no easy ride after a hump, it is the same pace for years).
Positive Software’s documentation team does very good work. Just a few weeks ago, we had an opportunity to work with cpanel. The documentation comparison is completely day and night.
Where as Positive Software organizes documentation for H-Sphere by category – system administration, administration, reseller, end user – there was no such organization for cpanel.
While Positive Software organizes documentation within each category where information from one document supports information from another document, not only was there no such organization for the cpanel documentation, but there were often end user contributed documentation, not all of which was in complete agreement with the other documentation.
The Positive Software support team has a good response time compared to others in the industry. While they do have areas to grow in terms of providing more detailed responses than “we fixed it,” they react promptly in comparison to others.
We moved to H-Sphere for our own hosting division several years ago. The move immediately saved us the annual cost of two employees – one in accounting, and one in support; and later on a lot more.