According to the e-newsletter that I just received from the SBB, it appears that normal GA/AGs and half-fare cards are also valid to get half-price fares on Austrian trains during June '08:

"Are you a regular customer who frequently makes use of your GA or Half-Fare travelcard? Then you too can take advantage of 50% off travel in Austria in June. So get ready to say a cheery "Servus" to all that this charming country has to offer. Continue."