My guess is that xbox 360 will be much tighter than the original in terms of security. Microsoft has lately been cracking down on modders pretty hard, so I assume they will try to prevent that with the 360.
Just because it's harder doesn't mean it won't be done, though. I suppose that supporting OSX on the 360 would require some work because of specialized hardware, but if hackers want it, they will eventually get it.
The above post is right though, Linux support will show up way before anything else. OSX is much less likely to appear.