Has he turned the corner?
He did, yes, but that’s not to say that he won’t turn another corner or end up in a blind alley. After three years of struggling in the major leagues, he even struggled in AAA in 2011. He started 2012 with the Tides and didn’t pitch particularly well at first; but gradually regained his effectiveness. When he was recalled to Baltimore this time, he continued to pitch well and was a key contributor to the Orioles’ postseason drive, although he didn’t pitch in the postseason itself.
The official line is that Tillman regained fastball velocity and consequently found a weapon against left-handed batters. While that may certainly be part of the reason, I also think that reduced expectations and success-driven confidence played a part. By the end of April, Tillman had been all but written off as a former prospect. Without being expected to be a savior, Tillman may have relaxed and stopped worrying about failure. Called up almost out of desperation, he pitched well and once he realized that he could get major-league hitters out, stayed relaxed.
Tillman has been durable; he hasn’t suffered an injury. There isn’t any reason to think that Tillman can’t continue pitching well; but there are certainly many more instances of pitchers coming up with one fluke season and then reverting to previous form.