This would be a great move IMO, and I sure hope it happens (especially if the 5th year is a team option).
While Lackey is not a true Ace, he is a workhorse, and incredibly consistent, and he has had postseason success.
A deal like this also opens up some other really interesting questions.
First, what do they do with Beckett? He would need a contract similar, if not bigger than Lackey's. Would they give it to him, or is Lackey ultimately his replacement, to help bridge the gap until Bucholtz or Kelly are ready to step up as the second Ace of this team with Lester?
Second, does this open up any deals for Bucholtz? Without Lackey, you could argue that its very risky to deal Bucholtz for a bat, because he is an important part of the rotation. But with Lackey, Bucholtz would just be the 4th or 5th starter (depending on what happens with Dice-K), and suddenly it becomes much easier to send him out in a deal for a big bat. Now, I think they would be hesitant to do this if they can't resign Beckett, but if he is locked up, they may let Buch go, since they have their top 3 locked up long term, and they have Kelly only a couple years away.
And finally, will this put the final nail in the coffin for Jason Bay returning? Sure, they could afford Bay if they really want to, but I am being more and more convinced that they are not enamored with him enough to give him the kind of money he is asking for. And with adding Lackey (and maybe a raise to Beckett), perhaps they will let him go, get a defensive guy to play left and (or first), and try to win with dominant pitching and defense. Then try to roll the money they would have paid Bay or Holliday to when there may be much better players in a year or two.