There are currently a few applications on Nokia S60 devices that let you use Skype, such as fring, iSkoot, and Skype Lite, but there has not yet been an official full Skype client on S60 devices yet. Skype Lite works with the cellular network and Skype Out to provide you with limited service. Today Nokia and Skype announced a partnership that will bring Skype to Nokia devices, starting with the Nseries and in particular the N97. This won't happen until the 3rd quarter of 2009.
While iSkoot and Skype Lite use cellular networks to give you limited Skype access, fring can be used now with cellular and WiFi connections to give you full Skype access. fring actually just won the Mobile Monday Community Award at Mobile World Congress. I like to use fring on my S60 devices because it also integrates Twitter, Facebook, and Last.fm in a single client.