A couple of ahem… hiccups have popped up for some users of VMotion after upgrading to vSphere. The Knowledge base team has been ultra-fast in responding to these, and two new KBs have appeared: VMotion stops working after upgrading to vSphere 4.0 (1011294) and VMotion stops working after upgrading to vSphere 4 with a Licensing [...]

Many customers are hitting a new bug in ESX preventing them from powering on virtual machines or VMotion them.  There is a published knowledge base article on this which reads>> Unable to Power On virtual machine with “A General System error occurred: Internal error” A virtual machine fails to power on, fails to leave suspend [...]

Here is another new and very useful article from our knowledgebase discussing the majority of reasons for VMotion errors. The online version can be found here: Overview of migration compatibility error messages.