Getting ahead of the storm
For those out there who haven't yet heard, VMWare is experiencing a problem (as of Aug 12) with licensing for ESX and ESXi. You can find out more here: http://kb2.vmware.com/kb/1006716.html
The crux of the issue seems to be that a recent update caused the license to expire, preventing any guest OSs from starting. A talked about workaround (see http://www.deploylinux.net/matt/2008/08/all-your-vms-belong-to-us.html) is to disable the NTP client on the host machine and roll the clock back a day or 2. Before embarking on this solution, let me add a word of caution:
The guest OS will take the host OS's time if you are using the 'time synchronization' option for the VMWare guest tool (as recommended by VMWare). This will cause any domain controllers running as a guest OS to roll its time backward. Now, VMWare has promised a solution to the problem by 12:00 PST Aug 13. If you can operate safely without these domain controller, it may be advisable to do so, as opposed to rolling back the clock.
I know this is a hot issue, so I will try my best to stay on top of any questions posted in the comments. Keep in mind, I have no knowledge of VMWare's systems.