That explains why they agreed to trade for the worst contract in the NBA (ESPN). Well done, Brooklyn. You kept your superstar (ESPN), and it appears that the acquisition of Joe Johnson pushed Williams to sign with Brooklyn (Yahoo).
This gives the Nets two max contracts. In addition to Gerald Wallace's deal (ESPN), the Nets have over $46 million tied up in those three players for the 2012-2013 season. That number will only go up. This means the Nets will not have the money to give Dwight a max contract. Even if they clear the rest of their roster, the ghost cap holds (cap holds for open roster spots when a team has less than 12 players) would take up about $3.8 million (ghost cap hold = rookie minimum = $474K and there would be 8). This leaves the Nets with less than $10 million in cap room.
With a salary cap around $60 million, the Nets must acquire nine 2nd round picks (2nd round picks eliminate the ghost cap hold), or they must hope for a surge in BRI (basketball-related income) to increase the salary cap.
The Magic could trade Howard to the Nets, but why would they do that? Howard cannot force a trade to the Nets because of the unlikelihood that he would sign with Brooklyn for a starting salary of under $10 million. Unless Howard shows a willingness to take a large pay cut, the Nets lack leverage in their trade negotiations with the Magic. Without that, the Magic can go out and make the best trade for them.
Compare the Nets potential offer with some other teams
Let us ignore draft picks over the next five years (unless they come from a third party) because any team that gets Howard will guarantee themselves 45 wins
Nets: Brook Lopez, Marshon Brooks, Kris Humphries for Hedo Turkoglu and Howard.
Hawks: Al Horford, Jeff Teague and a combination of expiring contracts (from Brooklyn and/or Devin Harris) for Howard and a combination of bad deals (say Turkoglu and Richardson).
Lakers: Bynum, Ron Artest and Josh McRoberts for Howard and Turkoglu
Clippers: Blake Griffin and DeAndre Jordan, Gomes for Howard and Ryan Anderson (depending on how his restricted free agency goes).
(If Ryan Anderson's contract is too big to make the deal work, then make it Griffin/Jordan for Howard.)
The Nets come in at a distant fourth. If I'm the Magic, I leap at the chance to get Griffin. They would have a marquee star to fill the arena and attract talent. Even a Lakers or Hawks trade yields the Magic a quality star (2nd or 3rd best player on a championship team) along with cap relief.
The Nets can offer the worst rebounding center in the NBA (in terms of TRB percent), a shooting guard who at best turns into Jamal Crawford (and I'm stretching...a lot) and a rebounding role player that is available in many free agent years. Now, this trade would make sense for the Magic if that poor rebounding center was Bob McAdoo. The Magic would have to want to fail as an organization to deal Howard for Lopez, Brooks, Humphries and a bunch of picks in the high 20s.
The Nets did what they had to do in order to keep Deron Williams in Brooklyn. Without him, the Nets would have stared at overpaying Brook Lopez and placing Gerald Wallace at the center of their brochures. However, in their attempt to keep Williams, they killed almost any chance of Dwight Howard becoming a Net.
Now they occupy the dreaded level of not being good enough to win a title and not being bad enough to get a high draft pick.
*Note: As Brandon Koay brought up in the comments, a Clippers trade involving DeAndre Jordan must wait until some time (or after) December. Since the Clippers resigned him as a restricted FA last December, Jordan can veto any trade for up to one year.