Yeah I've never heard of him either (though he was listed as a camp invitee), but supposedly he was a beast in the D-League. Here's info from the Celtics press release:JR 12/09/2011 10:36:00 PM Tweet
he Boston Celtics today announced that they have signed free agent center Greg Stiemsma. Per team policy, terms of the deals were not disclosed. Stiemsma, a 6'11"" center, last played for the Sioux Fall Sky Force of the NBA Development League. He appeared in four games for the Sky Force averaging 9.0 points, 10.8 rebounds and 4.5 blocks per game. He was named the 2010 NBA Development League Defensive Player of the Year after he averaged 9.1 points, 7.3 rebounds and a league-leading 3.7 blocks per game for Sioux Falls.There's been a ton of news today and in case you missed any of it, you can check out the 25 most recent posts here. In fact if you live the Celtics Life you're going to probably want to bookmark that page.