As my brother Pete wrote, the Steelers getting David DeCastro at No. 24 is a great result for them -- they had a terrific player fall into their laps, and he happens to play a position of serious need.
But perhaps this apparently-surprising development isn't so surprising after all. One mock draft after another had the Steelers taking Dont'a Hightower, to the point where Pittsburgh Newsdesk contributor Jordan Ruby and I had a running joke about the number of Hightower updates we were writing. The Hightower pick made a lot of sense in the context of those mocks, but everyone was making so much sense that there appeared to be a consensus that didn't exist in real life. In reality, everything after the first two picks was in flux, and there was a strong likelihood that a brighter light than Hightower would fall to the Steelers. I'm sure it helped that DeCastro happened to play a position where the Steelers need serious help, but it also wouldn't have been surprising to see the Steelers pick a player at a position of less obvious need if some other big-time talent had fallen.
Looking at the draft board, you can see how this happened, and no pick looks stranger than the Seahawks' surprising selection of WVU's Bruce Irvin, who I until recently figured would go somewhere around the third round.
Look for the Steelers to try to find additional offensive line help later in the draft. They'll probably also look for a running back and, if they can find picks that suit them, an inside linebacker or nose tackle.
For more on the Steelers, check out Behind The Steel Curtain.