The PDC Emulator is the operations master that will have the most immediate
impact on normal operations and on users if it becomes unavailable. Fortunately,
the PDC Emulator role can be seized to another domain controller and then
transferred back to the original role holder when the system comes back online.
Krzysztof PytkoSenior Active Directory EngineerCommented:
(out of points, please)
When PDC Emulator is down you can notice some minor issues like:
changed user password in the same time on more than 1 DC :) (specific scenario) then PDC holds actual time for this action to evaluate which action should take precedence.
lockout accounts, bad passwords and NT4 support. When you have no NT4 DCs in your domain environment then PDC is not big issue.
See here for how: http://support.microsoft.com/kb/255504