Skype in another series of great announcements, has extended the SkypeOut credit expiration policy.
Basically before, it was a pre-paid environment - buy 10 euros they expire in six months. Now everytime you make a call, you account balance is extended 6 months. So if your not a heavy PSTN user the 6-700 minutes of PSTN calling you get for 10 euros could last you all year, if not longer.
Short of having no expiration date (which in the pre-paid world is not smart business at all) this is about as good as it can get. It will be interesting to see if Skype can incorporate this never ending expiration idea, into other services they plan to offer, such as SkypeIn, in the future.