It was suggested in another thread that it might be a permissions problem. In my case, I don't think that's the case.
This is under ubuntu.
I run:
$ sudo swift --reg-filewrite
And I'm told
The information you have entered appears to be valid.
Thank you for purchasing a Cepstral audio distribution license.
You have successfully enabled saving audio to a file.
I see the license file here:
/opt/swift/voices/Allison-8kHz
Its permissions look like this:
-rw-r--r-- 1 root root 164 Mar 5 13:53 license.txt
As you can see, everyone can read the file.
Strangely, when I do the registration I still get the nag message sometimes. When It does stick, it only sticks through the next reboot.
Also, if I rerun the registration (sudo swift) although I'm again told that it was successful, the date on the license.txt file doesn't change.