This just in: KB948724 - "Changes are not committed when the Commit() method is called in SharePoint Server 2007 or in SharePoint Portal Server 2003" that was published yesterday (thank you KBAlertz for letting me know) affirms what I have written about a year and a half ago.
The Microsoft article describes a workaround (that me and the guys have already found, but I never got a chance to blog about.
Short story - if you try to do this:
It won't work.
Workaround - "To work around this problem, modify another property that is not a ChoiceList property. Then, call the Commit() method. The changes to the ChoiceList property will be committed."