I think (and hope) the new monk class will probably be featured in some DDI preview article, alongside the berserk, bard, druid, to be later put into PH2.
In my mind the monk should definitelly be Martial. Although Ki may feel a lot like some arcane and mystic force of nature, it's mainly an higher level of skill one achieves trough deep training of his body, breathing and concentration skills (think ie to Shaolin). DnD is not real world, so supernatural abilities coming from deep training is no problem, or at least this is how I look at it.
Talking about his role, I don't really see the monk fit well the defender. Given his sup bonus to AC and even a light armor, being a Martial Artist is all about speed, acrobatics and ease of movement, not absorbing enemy attacks. A monk could (and should) possibly be more of a die hard, compared to a rogue/warlock, but not as much good as a warrior/paladin when coming down to taking damage.
So I would say a monk would make for a good striker, with some twist. I'm thinking about powers that let the monk perform counterattacks, grab his enemy and throw him around, perform flying kicks while charging and other stunts, other than the classic ki-shot with stun effect or gattling punches.
