Hm. I may have spoken too soon. I've gotten three of those errors now...
I'm running with one of my wdc's set to do 32-bit transfers, instead of
the (I believe) default 16.

Also, I see something I believe is happening close to the addresses listed...
Grepped from /var/log/messages: 

Jan 29 17:16:39 acheron /netbsd: Data modified on freelist: word 0 of object 0xf05d0fd0 size 4 previous type UVM amap (0xfeadbeef != 0xdeadbeef)
Jan 29 17:16:49 acheron /netbsd: uhci_device_intr_transfer: not done, ii=0xf05a8f20
Jan 29 18:37:06 acheron /netbsd: Data modified on freelist: word 0 of object 0xf05d0f30 size 8 previous type UVM amap (0xfeadbeef != 0xdeadbeef)
Jan 29 19:07:41 acheron /netbsd: Data modified on freelist: word 0 of object 0xf060a690 size 16 previous type UVM amap (0xfeadbeef != 0xdeadbeef)

I'm rebuilding a kernel with the default wdc settings, just for kicks. If I
see the error again, I'll try this without any USB stuff, to see if that
makes a difference.

Actually... I'm gonna try w/ no USB but with pciide enabled right now, to see
how that does.

-- 
   Mason Loring Bliss   mason@acheron.middleboro.ma.us             E w i g e
awake ? sleep : dream;  http://acheron.ne.mediaone.net  Meerschweinchenkraft