We've got reason to believe that Skype will be incorporating additional "Push to Talk" features in the future. (See this press release from GIPS)
From the forums the discussion leads to always on and push to talk. Always interesting to 'hear" how users think about the product potential.

1) It makes Skype even more personal and the communication more seamless - why wait for the ring and the formal answer to start talking. Kind of like a voice version of instant messaging where instead of a message popping up a voice is heard. (This could be intrusive from some people which is why you could enable or disable some of your contacts as PTT'able). Even instant messaging would be boring if we had to ring the other computer before we started chatting.

2) It would turn every PDA (and computer as well) with Skype into a walkie talkie that works over the entire globe. How cool would that be!
forum.skype.com