Excelent idea.
I have quite a use for a similar system.
Im thinking, that the best method of what you sugest, is that once you check that check box, it becomes duplicated in 2 forums.
This probably isnt desirable, because if you edit the post in the forum, nothing will happen to the news post. Also with the comments.
Another option, would be to create a completly different table in the database, which stores the postid of each news post, and the details needed.
At the moment, such a system isnt so urgently needed by myself- however, the system i do need, is 'columns' as mentioned above for OPNews v1.5, or columns, cant remember what i called it
Your welcome to come to my board to discuss it all(*pimp* need more hits D: *pimp*)
So yer, atm im working on a columns system, which details are still fuzzy, and after releasing that ill help you out with this one, cause i can use it too(and i can imagine that they would be similar).
Give me about another 24/48 hours and i should be well underway!