This was posted on a different blog yesterday:
If a player is a free agent (not on any active roster), he can sign with any team right up to the last game of the season. However if, on March 1, he is still on an active roster (Like Joe Smith is on the Thunder right now), he would not be eligible for posteason play. "Off the roster" means being placed on waivers for 48 hours and no one claiming him.
So Joe Smith would need to clear waivers by midnight March 1. He would have to be placed on waivers by midnight tonight. What the Thunder could do is place him on waivers and hope someone claims him and the remainder of his contract by midnight March 1, and whoever claims him will have him available for the playoffs.
So Smith would need to be bought out today, and then placed on waivers tonight for him to be playoff eligible.
If that's correct, Smith would have to have been released before midnight last night to be eligible to play for the Cavs in the playoffs, and the article linked in the original post contained now news of that happening. Doesn't mean it didn't, necessarily -- and Smith could help secure homecourt advantage for the Cavs and then miss the playoffs, I suppose -- but at the moment there's no reason to believe that Smith is headed back to Cleveland.
Edit: just saw a reference on ESPN.com to "the increasing pessimism coming from rival GMs about Smith being waived in time (Sunday's deadline is 11:59 p.m.) to be eligible to play in the playoffs for someone else." So possibly good news -- but it also suggests there's still time, and that the person who posted the above may have been wrong (then again, this wouldn't be the first time someone on ESPN.com misunderstood the transaction rules, so who knows?).