mdb, is this the outbound or the return portion of your trip???

Setting aside the BUMP program, if your DTW--PHX flight is overbooked and you are a volunteer, the agent will likely be happy to just reroute you onto the DTW--LAX nonstop- my hunch is later PHX flights will be full, anyway.

If this is the case, the PHX--LAX HP flight will remain completely unused- which is no problem if you are ending the trip. If you have HP flights on a return trip, though, you should be able to explain that you were rerouted directly to LAX and won't use PHX-LAX, but still will fly LAX-PHX later (if that's your routing). NW will document in their system that you were rerouted-- you can even show your bump voucher as proof. In the worst of cases, talk to NW at LAX and they can likely put you on the nonstop back to DTW. When you are bumped, you really are granted a good degree of flexibility.

In a parallel situation, returning from Christmas I was bumped on DL out of MSP. My routing was MSP-ALT-LAX on DL then LAX-PHX on UA. The MSP agent was going to put me on the NW nonstop to LAX-- I asked to go to PHX instead, and actually, she never even verified the UA e-ticket. She filled out a carbon-copy FIM for MSP-PHX and I was on my way. No problems at all.

Good luck!!