Finally an update to this mess - the property has been sold to Verny Capital from Kazakhstan and a new operator for the hotel has been found:

The hotel will be operated by Ritz-Carlton and is scheduled to open Q2 2012.

Source: Die Presse (German) or Hospitality Industry (English)

This makes the market in Vienna even more interesting, with Four Seasons, Park Hyatt, Kempinski and Ritz-Carlton all opening in the next 2 years.