I believe when this thread was started (in 2005) the agreement between DL and NW/CO with regard to club members still only allowed NW/CO members into DL lounges if flying DL, and DL members into the NW/CO lounges if flying NW/CO.

The rules have since changed, and NW, CO and DL club members can now access any NW, CO or DL lounge at any time, even if they're not flying one of those airlines.