Don't think the Sonic Cruiser was ever a serious idea.
Problem for Boeing now is that the 747 is being undercut by their own 777 during this austerity period. They have a choice of ways to go. Firstly, a new aircraft (far too expensive and risky these days). Secondly, let the 747 die and produce 777 derivatives. Thirdly, develop a really worthwhile "old" 747/400 upgrade that is cost justifiable to soak up some of the many low time 747s available. Otherwise they are out of the "big" aircraft game.