>My suggestion:
>
>- If people have participated in a thread
>- And that thread gets moved to a forum to which they're not subscribed (so their messages will seem to vanish)
>
>Then, could those people get some sort of notification:
>
>"Thread {Thread Number} {Thread Title} has been moved to {New Forum}. You are not currently subscribed to {New Forum}. If you would like to continue to participate in this thread, please subscribe to {New Forum}".
>
>Another possibility would be to automatically subscribe those users to the {New Forum}, in that particular circumstance. That could be combined with a notification similar to the above - some people might get annoyed if they get subscribed to new forums without notification.

the bolded would include me if it happened often. Why not create a special "show thread owner participated in" option -
this overrides forum deselection if enabled, showing such moved threads without altering your settings automatically
on the move of threads totally under outside control.

And as "normal" threads in blocked forum are hidden, that won't happen too often
- which could be used to minimize added lookup cost.

A user might want to screen out a forum he participated recenlty totally - and he can so still by disabling this new option ;-)
Not knowing the data layout here this will cost more search cycles, but with corresponding indeces not too much I hope.

my 0.002€

thomas