Use <Unmap> and <Map>:
    bind . <Unmap> {puts "Someone just iconified me."}

More sophisticated programming recognizes "that [toplevel] [bind]ings
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 [WM]s the multidesktop is handled the same
as an iconify, so you still might get 'false' hits."