Drafting Bobby Carpenter at #18 might be a bit of a reach, but at the most, it's a half round reach. Bobby's generally graded as a late first early second round talent, around 8.95-8.99 using the Ourlads scale, which by the way is about the same range that Manny Lawson is rated by Ourlads.
I can name a few defensive tackles that were bigger reaches than Bobby or Manny would ever be.. Ryan Pickett and Dimitrius Underwood come immediately to mind. Casey Hampton of the Steelers was a one round reach that didn't turn out all that bad.
David.