He was hurt earlier in the year.
Here is the link to the article that I took this part from:http://web.minorleaguebaseball.com/news/article.jsp?ymd=20080323&content_id=372892&vkey=news_milb&fext=.jsp
Lefty Cole Rohrbough threw a bit early in camp, but has slowed down some in recent days as he continues to recover fully from an offseason ankle injury and some tenderness in his arm. With the ankle injury perhaps a contributing factor to the arm soreness, Kemp said the club just wants to be cautious. While Rohrbough still appears headed to Myrtle Beach, he may be a week or 10 days behind the Pelicans' other starters.
In a Q&A with John Manuel from BA, Manuel stated " He's on the Rome DL to start the season. Our latest info is that he had some shoulder soreness but it's supposedly minor and he'll be ready sooner than later. He's a great example of how the Braves have restocked their system even after the big Teixeira trade."
All of this info was from early Arpil so I am guessing his shoulder soreness was a bit more extentsive than originally thought(all speculation on my part from reading between the lines though). Hope this helps you out a little bit anyway.