The best explanation I've read of sharpness as it relates to aperture and depth-of-field is How To Improve Sharpness by Lars Kjellberg at photodo.com. It's an oldie and a goodie. In short:

Large apertures =
  • Shallow DOF, which is good if that's what you want
  • Less sharp because the large hole allows light from the edges of the lens, which is more prone to distortion

Small apertures =
  • Deep DOF, which is good if that's what you want
  • Sharpness compromised due to proportionately large percentage of diffracted light caused by such a small aperture hole (see Lars' article if you want a detailed explanation)

Mid-range apertures =
  • Mid DOF, which is okay if that's what you want
  • Best sharpness because the smaller hole means you are only using the best part of the glass (the center) but the hole isn't too small, which would allowing diffracted light to dominate what hit's the sensor