virtualbox.org

In NAT mode, all virtual machines get the SAME ip address ..
impossible to ping each other ...

Using the Virtualbox V4.2.1
Macbook retina MacOS X 10.8.2

That is the way nat works. read all about it in your users manual.

If you want NAT to run a different way you will have to run your own third party NAT software on the host (or in a vm).