Is it possible at the season 5 finale..both happened. That being Juliet hit the bomb causing the swan never to be built...but right before she could do that they time jumped to after 815 crash 2007. So as it is blowing up are survivors are in present 07. So it would makes sense that Desmond could very well be on 815 (if he really was) because his hatch was never built.
I think the sending them back to 2004 had some side affects...at least on Jack...he knows something is up