Start -> All Programs -> Administrative Tools -> Domain Controller Security Policy. You can ignore any errors about truncated strings ... gotta love one-button alerts. Then navigate into Local Policies: Security Options, and set the following:
- Microsoft Network Server: Digitally sign communications (always): DISABLED
- Microsoft Network Server: Digitally sign communications (if client agrees): ENABLED
- Microsoft Network Client: Digitally sign communications (always): DISABLED
- Microsoft Network Client: Digitally sign communications (if server agrees): ENABLED
- Domain Member: Digitally encrypt or sign secure data channel (always): DISABLED
- Domain Member: Digitally encrypt secure data channel (when possible): ENABLED
- Domain Member: Digitally sign secure data channel (when possible): ENABLED
Warning: This hint lowers your network's security a bit, use at your own risk.
[robg adds: I can't test this one, nor can I vouch for the impact it has on security. If anyone has any comments one way or the other, please add them. And yes, I know it's not really an OS X hint, but it's a hint to help OS X machines in a Windows-server world...]

