In his audio chat Thursday, Dave D'Alessandro was asked why the Cavs have clinched and the Nets haven't, despite the fact that the Nets are 1/2 game ahead of Cleveland. Dave thought it had something to do with divisions, but it doesn't:
Even though the Nets are 1/2 game ahead of Cleveland, they have the same # of wins (42). Chicago, the current 9 seed, has 40 losses, so they can win a max. of 42 games, which would tie them with Cleveland or NJ (if one of those teams lost the remainder of their games). Cleveland owns the tiebreaker over Chicago (swept the season series 4-0), but the Nets do not (currently 1-1) yet.
That's why the nets have not yet clinched a playoff berth. They will clinch with their next win or the next Chicago loss.