Did you forget about all the fancy CPU masking you did in version 3.x to get vMotion to work across different CPU revisions.  Well if you did you may run into this when you upgrade. You can reference VMware Knowledgebase article 1011294 when calling VMware support if the resolution does not work.
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<strong>Symptoms</strong>
<ul>
	<li><span style="font-size: 10pt; font-family: Arial;">VMotion fails after upgrading from ESX 3.x to ESX 4.0</span></li>
	<li><span style="font-size: 10pt; font-family: Arial;">You receive an error similar to:</span><span style="font-family: Courier New; font-size: x-small;">Unable to migrate from &lt;source server&gt; to &lt;destination server&gt;: The CPU of the host is incompatible with the CPU feature requirements of the virtual machine; problem detected at CPUID level.</span><span style="font-size: 10pt; font-family: Arial;">or
</span>
Host CPU is incompatible with the virtual machine's requirements at cupid level 0x1 register'ecx'
Host bits: 0000:0100:0000:1000:0010:0010:0000:0001
Required: 1000:0100:0000:100x:xxx0:0x1x:xxx0:x001
Mismatch detected for these features:
*General incompatibilities; refer to KB article 1993 for possible solutions. <span style="font-family: Courier New; font-size: x-small;">
</span><span style="font-family: Courier New; font-size: x-small;">
</span></li>
	<li><span style="font-size: 10pt; font-family: Arial;">This issue occurs after upgrading the virtual hardware in the virtual machines</span></li>
	<li><span style="font-size: 10pt; font-family: Arial;">A new virtual machine created on vSphere 4.0 migrates successfully</span></li>
</ul>

<strong>Resolution</strong>

<span style="font-size: 10pt; font-family: Arial;">The upgraded virtual machines may have some CPU masks applied which are causing the migration difficulties.</span>
<span style="font-size: 10pt; font-family: Arial;">To ensure that VMotion is successful:</span>
<ol>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Power down the virtual machine.</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Click the link to <strong>Edit Settings</strong> of the virtual machine.</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Click the <strong>Options</strong> tab.</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Select <strong>CPUID Mask</strong> under <strong>Advanced</strong> .</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Click <strong>Advanced</strong> .</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Click Reset All to Default.</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Click <strong>OK</strong> .</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Click <strong>OK</strong> again.</span></li>
	<li>
<span style="font-size: 10pt; font-family: Arial;">Power on the virtual machine and migrate.</span></li>
</ol>
