Trying to run the Group Policy Results Wizard in SBS 2003 from the server to a remote PC gives the following error message:

Group Policy Error

Failed to connect to <computername> due to the error listed
below. Ensure that the Windows Management
Instrumentation (WMI) service is enabled on the
target computer, and consult the event log of the
target computer for further details.

Details: the RPC server is unavailable.

Needless to say, WMI and RPC server are both running on the target computer and the event log contains no entries.

The Microsoft solution is to enable "Allow Remote Administration Exceptions" in the Small Business Server Windows Firewall GPO (Computer Configuration | Administrative Templates | Network | Network
Connections | Windows Firewall | Domain Profile).

Naturally this doesn't help any.  A helpful post by DonL on Microsoft's public SBS newsgroup gives the fix:

The RPC port 135 is being blocked - by the Microsoft Firewall Client for ISA
2004. To unblock, do the following:

Go to ISA Server Management
Browse to "Firewall Policy"
Select "Edit System Policy"
Browse to "Active Directory" under "Authentication Services"
Uncheck the box titled "Enforce strict RPC compliance"
Click OK
Apply the new settings