We know that we've lost for sure:
The Times They Are A' Changin'
Apple Tree (it's a limited engagement)
High Fidelity
I don't think Martin or Butley will hold out much longer. Company is a wild card, it could go either way.
Older show wise, it wouldn't suprise me to see Producer's fall, nor would it suprise me to see Tarzan go either. (Granted, this is not based on ticket sales, just my own feelings)
http://www.youtube.com/huskcharmer