No, you do need to run a converter on them to move them across supported platforms.
Part of the differences that can cause issues are the certain hardware, for instance you have USB2 support in WS6 but not in other products.
Conversion doesn't take too long (of course that depends on the size of the VMDK and the power of the machine doing the work).
One of the nice things with converter tho is that you can also convert MS VHDs so its worth having.
Simon
The real world is not about exam scores, it's about ability.