There is, apparently, a code that can be entered into the D1 to prevent the face from opening. It appears, from my talking to Pioneer customer service, that this is used in situations such as when the D1 is (was!) on display in a car audio store. The lock code would prevent anyone from potentially swiping either the navigation DVD or any CD that was in the unit. This code is supposedly entered by pushing some combination of buttons simultaneously.
To follow-up---I DID find an E-bay retailer selling the service manual as a PDF download. The code is apparently entered, according to the manual, by holding in the reset button while simultaneously pressing the AV and INFO buttons. I tried this w/ my D1, and it didn't solve my problem. But for some strange reason, when I went out the next (yesterday) morning, all was working normal again.