iterm + tmux isn't allowing me to scroll up with the mouse
The recent release of tmux 2.1 changed some of the configuration options. This bit of code (taken from here) restores the previous functionality:

bind -n WheelUpPane if-shell -F -t = "#{mouse_any_flag}" "send-keys -M" "if -Ft= '#{pane_in_mode}' 'send-keys -M' 'select-pane -t=; copy-mode -e; send-keys -M'"  
bind -n WheelDownPane select-pane -t= \; send-keys -M 


Add/place these in the ~/.tmux.conf file

Apply with tmux source-file ~/.tmux.conf