I say a ban from MM means a ban from the forums, but not the other way around. This would keep pricks from whining about their bans in the forums. However, as stupid as some of these trolls are here, I don't think they should be banned from MM. Personally, I would love to see that happen; I have zero tolerance for stupid people. However, they did pay for the game, which means as long as they don't violate Bungie's or XBL's rules in MM (not on the forums), they're entitled to the ability to play on XBL.
The two services are indeed connected, but not enough to warrant such a punishment. They're connected arbitrarily, through stats and screenshots and such. Being a douche on the forums doesn't affect anyone's gameplay experience on Live, and vice versa.
See, I'd love for the guy that posted "newb pwners only!!!!1" to be banned from MM until 2552, but I know that won't happen. *sigh*
[Edited on 10.28.2007 4:54 PM PDT]