Okay, I figured out the problem (at least, for me).

There is a section of code in wdc:wdcprobe() right in the beginning:

        /*
         * Sanity check to see if the wdc channel responds at all.
         */

        bus_space_write_1(chp->cmd_iot, chp->cmd_ioh, wd_sdh,
            WDSD_IBM);
        delay(1);
        st0 = bus_space_read_1(chp->cmd_iot, chp->cmd_ioh, wd_status);
        bus_space_write_1(chp->cmd_iot, chp->cmd_ioh, wd_sdh,
            WDSD_IBM | 0x10);
        delay(1);
        st1 = bus_space_read_1(chp->cmd_iot, chp->cmd_ioh, wd_status);

        WDCDEBUG_PRINT(("%s:%d: before reset, st0=0x%x, st1=0x%x\n",
            chp->wdc ? chp->wdc->sc_dev.dv_xname : "wdcprobe", chp->channel,
            st0, st1), DEBUG_PROBE);

        if (st0 == 0xff)
                ret_value &= ~0x01;
        if (st1 == 0xff)
                ret_value &= ~0x02;
        if (ret_value == 0)
                return 0;

#ifdef'ing this code out made my CD-ROM drive start working.  Jonathan,
can you see if this works for you?

Manuel, do you have any ideas?  I will agree that my CD-ROM drive is
cranky, and I'm willing to quirk this code ... but should it even be
there?  I think that other OS's IDE drivers don't do this.

--Ken