So, Peter Hutterer is my new best friend. Peter has found and patched the bug in the X server that was causing the failure of negative transition triggers in XSync. For you and me, that means that XIDLETIME now works as expected, and gnome-session and gnome-power-manager do the right thing. This means no more random blanking when typing.
The important patch for the xserver is here and I urge all distros to cherry-pick this commit into their stable branch, as this affects a lot of users. Hopefully this is the end to the saga of accidental screen blanking.