A runaway process on SBS2003 decided to fill up all the disk space on C: in the early hours of the morning. The fallout from this was the System log was corrupt and the SBS license data was reset to the default 5 CALs.
The System log was easy to fix - reboot the server.
The SBS license data was an absolute pain. I'd never run the "Back up licenses" utility in the Licensing section of Server Management. Microsoft have KB article 888818 discussing this, which is either re-enter the licenses, restore the C:\WINDOWS folder or restore a backup of the licenses.
The first wasn't an option as I was offsite and the person with the key to the safe wasn't in. The second was just not viable - why Microsoft couldn't specify which file/folder needed restoring I don't know. The third would have been OK if ever I had run it.
After much stuffing about I found that the SBS2003 licenses are kept in the licstr.cpa file in the WINDOWS\system32 folder. Thankfully, Microsoft actually keep an automatic backup of this in autolicstr.cpa. The simple process was to stop the License Logging Service, rename licstr.cpa to licstr.cpa.old, then copy autolicstr.cpa to licstr.cpa. After this I started License Logging Service and used Server Management to confirm that the licenses had been restored.