Its on a diffent domain yes. But I have addressed that by adding all of the esxi hosts to the local hosts file. Name resolution is working fine.
I had a similar issue on a 2003 server, and dropping the version back from 5.1 to 5.0.1 fixed the issue. However as this box has GPT disks, thats not possible.
Really would like to get to the bottom of it, its an odd one.