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 server is not configured [...]

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 mode, or [...]

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.

A common problem I see is when VMotion fails at the 10% mark with a timeout. You might see a dialog popup which says-
Operation timed out
Tasks: A general system error occurred:
Failed waiting for data. Error 16. Invalid argument
The VMware Knowledgebase writers have assembled a great list of possible reasons for this problem and I [...]

Ok, here is the KB article discussing the problem where the CPU usage of a virtual machine might increase significantly after VMotion migrates the virtual machine in a cluster with DRS enabled. As a result, the performance of the virtual machine might be degraded.
“Starting with ESX Server 3.5 and VirtualCenter 2.5, DRS applies a [...]