Hello. I currently have 2 client accounts on my installation of ISPConfig 3 (From SVN lastnight), each have their own Web space. However, I've noticed that one account can register a DNS entry for a domain name that's linked to the other account. I'm not sure if this is a bug, desired feature, or being worked on but from what I can see, the only real requirement is that the domain name is valid, and that the exact entry doesn't already exist. There's nothing to stop anyone stealing subdomains from anyones domain on the server.
Other than that, it's looking very impressive. It's clean, easy to use, quick and responsive. Great job!