GPS altitude is a useful back-up, but that is all
Since 1st October EGNOS (European equivalent of WAAS) has been fully operational and certified for "safety critical applications such as flying aircraft". It is accurate to 1 to 2 meters horizontally and 2 to 4 meters vertically so ideal for precision approaches for which it has been designed. (I believe the first trial precision IAP [LPV] is underway at Heathrow).
ESA - Navigation - The present - EGNOS - EGNOS ?Open Service? available: a new era for European navigation begins today
I agree that it may not be much use for relative separation as yet but plenty of other applications such as cloud breaks and approaches !
Drambuster