CoolP,

"But my main question still remains. What's preventing you guys from updating the INS when some DME actually IS in range?"

I am trying so hard managing  DME update. In ten updates, I succeed  only one. :D
It looks like DME update instructions are not clear to followup. Here are some items missing  from Steps:

Steps are
1) tuning that  (VOR/DME) station
2) switching the CDU to DME update mode (Right click on 7+9 Key)
3) entering the coordinates of the tuned station

VOR Altitude insert, 3+9  then 2 key

4) checking, if the rough distance (shown on the CDU display) matches the DME measured one

Turn knob to Way point then  Press Way point change +Press 1 and insert

5) starting the update (Green light should be ON)

Sanal