If you get the VT-x is not available error when trying to install or run Ubuntu under OSX in Sun’s Virtualbox, you can fix this problem by editing the the xml file for the VM.

With Virtualbox NOT RUNNING, first go into the VM’s directory.  On my computer its in:

/Users/john/virtualbox/Ubuntu

Then edit the xml configuration file.  Mine is: Ubuntu.xml

Look for the entry:

<HardwareVirtEx enabled="true" exclusive="true"/>

Change the “true” to “false” like so:

<HardwareVirtEx enabled="false" exclusive="false"/>

Thats it.  Start up Virtualbox & run Ubuntu.