Ok; then it must be my headset. I got one free when I bought the phone, so I'm thinking it's one without many "features"...it's a NoiseHush N500.
I experimented a bit with it today, actually, after I posted that. I discovered that if I press the button on the headset, my phone will say "please say a command"...but then the GPS will talk to me. Actually, the other phone sounds will come through as well.
Problem is, the voice command prompts will continue in the background, and then give up on me and shut down.
I think I'm going to do some searching around for threads about this Nuance voice command deal...it's pretty confusing and lame so far.