VMconverter does Multi-Vendor V2V conversions:

This was an awesome tip from the great VMETC.COM blog!
See this link for the full article!

ESX 2.x
ESX 3.x / ESXi 3.x
Virtual Server 1.x or 2.x
Hyper-V
XenServer
any of the hosted desktop products including (but not limited to) VMware Workstation and Fusion, Microsoft Virtual PC, and Sun xVM VirtualBox.

All that needs to be done is install VMware Converter directly on the VM’s operating system, and then make sure that the source VM has network connectivity the target ESX host. Once you’ve worked those hopefully small details out, VMware Converter is basically oblivious to the fact that the source is not physical hardware and conducts the migration as if it was a p2v. Just choose “physical machine” and “this local machine” as the source type options when configuring an “Import Machine” Converter job.