Dmytro let us know about his latest BlackBerry PlayBook app called Taki. Taki is a native SIP softphone VoIP client that can be used with multiple VoIP services. This allows you to make calls from your PlayBook if you are subscribed to a VoIP service that supports SIP (here are their recommended ones). According to the dev this can also be used to talk to Skype contacts if you have a Skype SIP profile or use a SIP provider that provides an interconnect with Skype.
I have not had a chance to try out Taki myself but check it out for $0.99 in App World or at the project website. Keep in mind this is the first release of Taki so expect some bugs. Let us know what you think if you get it to work.