Comment 8 for bug 237721

In my humble opinion, neither gdm nor kdm (or xdm) alone can provide this feature as some lower level support is necessary to implement a true SAK (linked to some hardware interrupt).
By the way, in some sense, this secure attention key already exists: it is CTRL-ALT-BACKSPACE, which will kill and restart the X server, and hence induce a session close and gdm/kdm/xdm restart. It is also certainly possible to remap CTRL-ALT-DEL to trigger a similar behavior. I'd suggest inquiring into these obscure system settings before submitting a wishlist.
Anyway, the most difficult task is certainly to teach users to hit CTRL-ALT-BACKSPACE (even security conscious ones) *before* typing their login/password... ;-)
Maybe we should simply add a new message to the login manager - if the marketing departement agrees...