Today, Skype released Skype 5.3 for Mac OS X, with a number of fixes and some notable improvements, including HD video calling. In addition to an improved user interface, Skype 5.3 will feature support for the new Mac Lion operating system and be backwards compatible with existing Mac OSes. To receive clear HD video calls on your Mac, you'll need an upload/download speed of 1.5Mbps.
Check out the left (non-HD) vs. right (HD) video calling example Skype gives: