USRobotics makes a cheap (€25, $30) Skype-compatible USB phone, but they don't provide a Mac driver.
However, the OEM has put out a beta Mac driver and it works! You can download it from this page. You run their Skypemate.app at the same time as Skype itself. Don't forget to set Skype's Preferences to use the phone for input and output; it shows up as "USB Audio Device".
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060113053515443