Comment on: "The Casimir force on a piston in the spacetime with extra   compactified dimensions" [Phys. Lett. B 668 (2008) 72]
  We offer a clarification of the significance of the indicated paper of H. Cheng. Cheng's conclusions about the attractive nature of Casimir forces between parallel plates are valid beyond the particular model in which he derived them; they are likely to be relevant to other recent literature on the effects of hidden dimensions on Casimir forces. 
S. A. Fulling and K. Kirsten