Skype dumped GIPS about a year ago, and as previously reported here on VoIPWatch, their audio seemed to be improving since the switch.
Now that I've been working from Europe for three weeks, while I've been working out of hotels I've been making extensive use of all types of mobile and laptop software for communications ranging from CounterPath's Eyebeam that works incredibly well with client Junction Networks onSip platform, GizmoProject, Skype and SightSpeed, I have to admit that Skype has really improved their off net audio quality, especially when it comes to calling mobile phones in the USA. At all times my connection has been WiFi, also providing additional validation to my concern that both DSL and Cable networks in the USA are not that "modern" and while good for non-real time data service, not very good for voice or video. To that end, all of my SightSpeed and Skype calls have been much "brighter" while here in Spain, where Telefonica has pretty much control of the network. I've had the same experience in the past in Portugal where PT pretty much has the access sewn up.
The quality of calls to the mobile users back home was always one of my concerns and pet peeves, but over the past few months that quality has gone way up. Sadly, this is coming near the end of the term of my Skype Unlimited Plan that expires in January.