Regression is in Tk HEAD as of Feb 13, 2011.
Regression also appeared between 8.5.8 and 8.5.9.
Code to reproduce : http://paste.tclers.tk/2356
It appears that
1. Cascaded torn-off menus will not show up anymore when selected with the ------ line;
2. Non cascaded torn-off menus will not show up if their "parent" toplevel is withdrawn.
Looks OK with Linux. XP issue.