The events of First Contact should technically push Enterprise in to an alternate reality. With the Borg Drones left over in the artic region of Earth and all. Make sense really, the events of FC creating an alternate timeline that is subtly different but close enough to the original timeline where the first contact with the Borg was 'Q Who' and not ENT's 'Regeneration'.
That's what I've been saying all along! Enterprise being set in an alternate timeline because of what happened in First Contact would be a logical way to explain away all the discrepancies between Enterprise and TOS/TNG.
I'd even go a step further and say that Star Trek XI is a continuation of the alternate timeline that Enterprise happened in. This would be the only way I can think of to explain the differences to the original timeline in that movie. The differences that were not caused by Nero.