It appears that Skype are unhappy over the use of the Skype API's and the interaction of Nimbuzz’s voice call service (which is similar to the Skype Out service that allows a computer to call a landline or mobile through a mix of regular telephone systems and VoIP). This is an area where both companies have an income stream that supports the free elements of the application.
Skype’s side of the issue is based around quality of service, and on their reading of the terms and conditions of the API licence that Nimbuzz used, Nimbuzz are breaking the API T&C’s and the End user Licence Agreement (EULA). While they have offered to open discussions with Nimbuzz, they’ve heard nothing official from them since early August (quotes at GigaOM).
The biggest looser in all this is the end-user. Skype will point out they have a Symbian Client so there’s no loss of functionality; Nimbuzz, will point out the benefits of multi-platform IM in one application, on multiple mobile OS platforms (including Symbian, Android and iOS) and that Skype aren’t being “fair"."
And yet again, IM fails to get any traction in the mobile mindset because it continues to trip itself up so publicly with issues like this.
Nimbuzz has blogged about the latest update here.