This question of what happened to the vCenter Converter plug-in has been coming up a lot lately in the communities, and here are some official/unofficial references about what happened.
(1) The vSphere 4.1 release notes from July 13, 2010 announced that "VMware vSphere 4.1 and its subsequent update and patch releases are the last releases for the VMware vCenter Converter plug-in for vSphere Client. VMware will continue to update and support the free Converter Standalone product, which enables conversions from sources such as physical machines, VMware and Microsoft virtual machine formats, and certain third-party disk image formats."
"With the release of vSphere 5.0, we will no longer provide vCenter Converter Integrated however, vCenter Converter Standalone will continue to be available as a stand-alone, public software binary. We will continue to provide support for vCenter Converter Integrated on the existing vSphere platforms per the VMware support policy and we will continue to provide vCenter Converter Standalone as a free download. You can find out more about vCenter Converter Standalone here: http://www.vmware.com/products/converter.
VMware Product Management"
(3) The vCenter Server Installer application does not list the vCenter Converter plug-in, and the media does not contain it.
The standalone version of Converter 5.0 was released on September 8, 2011 and may be downloaded at https://www.vmware.com/tryvmware/?p=converter.
Thanks for reading,