Use <Unmap> and <Map>:
bind . <Unmap> {puts "Someone just iconified me."}
More sophisticated programming recognizes "that toplevel bindings apply to every child widget, so you should also test that the widget is a toplevel" and moreover that "Unmap will also happen when you switch desktops, so you have to ... test the state" (in the words of Donald Arseneau ...):
bind . <Unmap> {unmapper %W} proc unmapper W { if {[string equal [winfo toplevel $W] $W] && [string equal [wm state $W] iconic]} { puts "Minimizing $W." }
Bruce Hartweg adds ..., "Also note that with some WMs the multidesktop is handled the same as an iconify, so you still might get 'false' hits."