There is a command line method for locking the screen, see here:
http://talk.maemo.org/showpost.php?p=209444&postcount=6 .

dbus-scripts can run a script on many system events, one of them is home key pressed.

You can either config dbus-scripts by editting its configuration file, or use dbus-script-settings from extras-devel.

If you don't mind pressing two keys (like home and then dpad left), look at http://talk.maemo.org/showthread.php?t=28004 .