Sold in several versions -- some with smooth leather skins, some with "perforated" leather, and some with "dangle" or "clasp" metal hooks -- Case Mate's Signature Leather Case for 3rd Generation nano ($25) is a simple leather holder for the video-ready iPod nano. Highly similar to Uniea's earlier U-Suit for iPod nano aside from the texture of Case Mate's Napa leather, which is nicer, the Signature Leather Case provides coverage for much of the nano's…