Subscribe to thread is a good idea, but I suspect it may be too evil with database hits.
How would you suggest it be done? PMs would just not work... Urm...
Maybe just make the little yellow "new post" star change colour or flash or rotate or something if a subscribed thread has been added to, and maybe change the colour of the "forums" link up top.
Come to think of it, that would only be one extra instruction per entry in the forums, max, so maybe it wouldn't be a database killer...
*shrug* Meh, it's up to Brad to decide.
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