I'm in a similar boat.. I'm trying to recreate Squad Leader by Avalon Hill in a browser. I think Hasbro might own the trademark, but I don't think copyright applies to game rules:
The ideas in the rules cannot be copyrighted, but the specific text of the rules can be (see http://www.copyright.gov/fls/fl108.html). Also, game mechanics can be patented.
http://www.battle-board.com/minisl/
If they send a cease and desist I guess I'll just take it offline. I don't think there are any damages in my case, but I'd still hate to be sued!