There's no realistic way in my mind that Lebron could be a Celtic. We don't have the cap room to sign him, which leaves us with one of two options: a sign-and-trade, or signing him for the MLE.
In a sign-and-trade, we'd have to trade Rondo and probably a resigned Ray Allen to even get Cleveland's attention, and I doubt they'd ever bite on that.
I doubt Lebron would agree to the MLE, either. In order to gain Bird rights, he'd need to sign a three year deal. He'd be giving up roughly $10m+ each year of the deal, meaning he'd have sacrificed in excess of $30 million in salary before he could sign a max deal (with us) again. I just can't see it happening.
I think Lebron's realistic options are Cleveland, New Jersey, the Clippers, the Knicks, and the Heat. Outside of those teams, I don't forsee anybody else landing Lebron.