'stsadm -o canceldeployment' when a solution deployment is already under way

If a SharePoint solution deployment fails, yet the timer job is created and not running, you will be unable to redeploy or retract the solution. The retract / deploy buttons will not be present in Central Admin and stsadm will declare that a "deployment or retraction is already under way for the solution". You can find yourself in this state if the “Windows SharePoint Services Administration” service is stopped when you first attempt to deploy, but even after you realise your mistake and start that service, the solution deployment is still stuck.
The answer is hidden in that long list of stsadm operations:

stsadm –o canceldeployment –id

This command will cancel the timer job that is meant to deploy the solution. Once it has been cleared, you can deploy successfully. Obtain the id parameter from the url to the appropriate deployment timer job in the Operations -> Timer Job Definitions page