The Nets have a roster opening that they've kept ready since Josh Childress asked to be waived. Starting Friday, they'll have to start thinking seriously about filling it. March 1 is the so-called "second trade deadline." The basic rule is that if a player isn't on an NBA roster --or waived-- March 1, teams can sign him and he'll be eligible for playoff rosters. Rosters are submitted to the league office one day after the regular season ends, meaning April 18. So, a player waived by Friday can still be eligible to play in the post-season. Agents for players who want to get onto a playoff will often ask to be waived or bought out by March 1. The process is already underway. The Magic has waived Hakim Warrick, the 30-year-old power forward. The Wizards will likely waive Jason Collins, the ex-Net, and Omri Casspi, the tough-nosed Cavaliers forward, reportedly asked for a buyout Thursday so he can join the Grizzlies or Rockets. Then, on Saturday, he suffered acute appendicitis and was rushed to an Orlando hospital.