For some reason enabling outgoing DomainKeys signing on my Virtualmin GPL box requires me to do it twice for it to show "Yes" in the "DomainKeys Identified Mail" section. Doing it once enable the records in the DNS files and starts the dkim-filter service, however if I go back to the DKIM section the "Signing of outgoing mail enabled?" shows "No". I have to click "Yes" again, save the options, and then it shows "Yes".
This appears to be an issue with only Firefox 4 Beta 9, as using Firefox 3 enables/disables it properly. This does not appear to be a caching issue with the browser however, as when enabled in Virtualmin the first time with Firefox 4, it shows No in both Firefox 4 and Internet Explorer. The DNS options are made and the service is running however.