I just watched Enterprise: Season 2 Episode 23 "Regeneration"
If you haven't seen it, it involves the finding of 2 frozen Borg drones in the Arctic Circle who were left over from the Borg invasion seen in First Contact.
At the end of the episode it was revealed that these Borg drones sent a sub-space message to the Delta Quadrant giving details of how to locate Earth, then goes on to say that a sub-space message would take 200 years to travel that far so they shouldn't expect an invasion until at least the 24th Century.
I liked this episode, but it suggests that the Next Gen episode where Q initiates Star Fleets "first" encounter with the Borg was not the reason that they decided to assimilate Earth.
If this were the case then Q probably knew about this impending attack and he sent the Enterprise D to the Delta Quadrant solely to prepare the humans of the era to learn how to defend themselves against the Borg.
A lot of people didn't like Enterprise and said it messed too much with what they see as Star Fleet continuity (especially regarding Klingons) but it was episodes like this that I found made the series really enjoyable.