The line's whatever you think it is. Personally, I see the title as ironic. There's no line and there's arguably not even lines - everything's a spiral and to suggest otherwise is too basic a viewpoint.
Alternatively, there's a few moments in the game which are perhaps the most obvious contenders for a definitive line being crossed, or you might think that 'the line' pertains to the idea of heroism becoming narcissism.
And because I am unafraid to do some self-promotion, I have a review of it here (beneath the Bastion review): http://www.escapistmagazine.com/forums/read/326.386789-Woodsey-Weviews-vs-The-Summer-Aftermath-2012-Part-3
Fraser Greenfield said:
I'm wondering if its worth getting, I mean I spent a considerable amount of my childhood in Dubai. Seeing it in ruins might bring back some nostalgia...
I would have thought it's a little too ruined to be recognisable, but then I suppose it would depend on how accurately it's modeled on the place. I suspect they haven't done it down to the street-name. Definitely worth a purchase though, especially as it's now going for cheap in a lot of places.