Regardless DB should be able to carve out space where cross platform is important, but I can see their market size shrinking when Apple and Microsoft really start pushing their cloud storage solutions. Unless of course they get to a Facebook like level of critical mass.
I'd guess that most Dropbox users share something (though I haven't seen statistics on that) so they are probably less threatened by vendor-specific solutions than you might think. I suspect Dropbox is more threatened by services like Amazon Cloud Drive (especially since Amazon is an infrastructure provider for them) than they are by anything Apple or Microsoft has planned.
I think the opposite. If HN readers tastes' are similar to the average founder in the Valley, they are vastly more likely to use Macs and iPhones together than the average real world user, who is much more likely to use Windows/iOS or Windows/Android.
I wasn't anywhere near the only one who had those kind of cross-platform needs. My program was a real mix of Windows and OSX users with a few linux users for flavor.
Pre-installing on PCs is possible, though my quick Googling didn't turn up anyone doing it. Pre-installation on Apple devices is unlikely, of course.