Eliezer raised an important privacy issue in a comment on my previous entry. One which I'd never really thought of in too much depth but one that is entirely valid.
Having looked into the issue of Plus account holders 'Friends Only' diaries suddenly becoming public when I run the expiry scripts for the first time in ages, I can now confirm the following;
No-one's diary will suddenly become public after a Plus account expires.
However, when you next try to customize your diary, the option for Friends Only diary will be greyed out and the tickbox will NOT have a tick in it. If you go ahead and save any changes, your diary will then become public (unless you chose to tick the private diary box).
I think that's a reasonably OK thing to do – please advise me if you disagree. On privacy issues I am (unlike Facebook ;)) always willing to hear other people's points of view and take them into account. Privacy is a serious issue.
P.S. The keen eyed amongst you may have noticed that this diary was in fact marked as Friends Only for a short time. I was testing how the system dealt with expired Plus accounts