This is probably horribly unrealistic.
The Hawks, like the Pacers, are reportedly losing a ton of money.
Let's assume they re-sign Joe Johnson to a big contract this summer. They may be looking to make a move to clear cap space after the 2011 season.
Here's my proposal:
The Hawks get a big man who will fit well with both Horford and Smith and clear about $23 million in salary over the next 3 years.
The Pacers get the younger, more talented player in the deal. Williams may, or may not, have more talent than he's shown so far. He's always been 3rd fiddle, at best, in Atlanta. He's only 23. It's quite possible he turns into something resembling the #2 pick. Even if he doesn't, he's not terribly overpayed as is.