I created a new workspace before starting any changes – I am not sure this was necessary. I uninstalled the Subversive plugin, and removed the Subversion directory that was left over in C:\Documents and Settings\…\Application Data. I restarted MyEclipse, and reinstalled the Subversive plugin. I checked out a project from a repository, made a minor change, and when I committed it, my correct user name appeared in the Author field. I switched to my original workspace, cleaned the project, and committed a change with the correct results again.
Thank you for your help.