In short, I wouldn't make any big changes in my message. Obama's core message, the change he wants to deliver, is "One America." So whatever he says and does at this point should be focused on that.
I'll be blunt: there's nothing wrong with being a uniter vs. being a divider (although by making the point, you're being a divider of sorts). That was why Obama's opponents pounced on the "bitter" comments -- because they wanted to point out that Obama was really a divider, was really just another hack (like, um, them).
No, there's nothing wrong with being uniter -- as long as you're genuine and work to find common ground instead of going for 50%+1 and pulling up the drawbridge. That was one of the first big mistakes Bush made after being elected: ditching the majority of people who didn't vote for him and governing as though they had.
[UPDATE: Today, Obama announced plans for a massive 50-state voter registration drive -- something only he can pull off because of his well-funded campaign. THAT'S how you build a party for the future.]
The unity message is a good one and fundamental to Obama's appeal. it was even the theme of his groundbreaking speech at the '04 convention. I think people want to hear more of that.
If anything, he needs to sharpen the picture of his opponents. Every political campaign needs an "us vs. them" message. Obama should talk about those who want to divide us. Because we're still in the primary season, he should talk about those (translation: Clinton) who want and need to divide the Democrats from each other. And he should talk about why they want that: the experienced hands have a stake in the status quo and will do -- and say -- anything to preserve it. Of course they'll fight Obama. And if Obama can continually point that out, he'll stay ahead.
Obama should talk about building the party for the future vs. relying on a "Kerry + one state" electoral strategy. He should talk about expanding the base -- not just shifting it to the right (hello Lieberman). He should talk about bringing in new, young voters (instead of relying on the just the older ones).
If the supers want math, talk about how his electoral college vote-ceiling is much higher than Clinton's. Talk about how down-ticket candidates get a boost from Obama and just the opposite from Clinton.
This message has the virtue of being transportable into the general election campaign. McCain (like Clinton) is one of the "experienced" Washington hands who think Obama needs more "seasoning." Like Obama has said many times, they want to "boil all the hope out of [him]."
People "get" that. They want something new, they want to turn the page. They're tired of fighting. Obama should talk about winning without fighting -- after all, we just got done with eight years of doing nothing but fighting to win (and then losing in the end). People are exhausted. They don't want to continue to go over the same old crap, again and again.
His message is straightforward: if you want the same old thing, vote for the same old people. But if you want change, you have to vote for the new guy.
If you want to get back to One America, vote for Obama.