New VMware Workstation 7.1.1 allows you to run ESX as a guest OS

Considerations for Running an ESX Guest
Evaluate the following aspects when running an ESX guest using VMware Workstation:

* ESX in VMware Workstation works only on processors that support Intel EM64T with VT-x or on the AMD64 Family 10H and later processors with AMD-V.
* ESX 4.0 guests must be configured with two or more cores. VMware recommends that the host system should have at least as many cores as the guest.
* Only 32-bit guests can be installed to run as nested virtual machines inside an ESX guest. These virtual machines can only be configured to use binary translation.
* This feature has been tested on only a limited number of host operating systems, including RHEL 5.2 Desktop and Server editions, Windows Server 2003 R2 SP2, SUSE Linux Enterprise Server 11, Ubuntu 9.04 Desktop Edition, and Windows Vista SP1.
* The nested 32-bit guest operating systems that have been tested are RHEL 5.4, RHEL 5.3, RHEL 4.8, Ubuntu 8.10, Windows 2003 Advanced Server, Windows Vista, Windows XP SP2, Ubuntu 9.04, and CentOS 5.3.
* ESX instances should not be compared to ESX running natively because all operations pass through VMware Workstation and the host operating system, which significantly degrades performance.

Source:
http://www.vmware.com/support/ws71/doc/releasenotes_ws711.html