Cannot mount cdrom: "device busy" error on 5.1-RELEASE-p2

I'm hoping this will be a simple question to answer, so here goes. 
Whenever I try to mount a cdrom I get the following:

borges[141]% sudo mount -t cd9660 /dev/acd0 /cdrom
cd9660: /dev/acd0: Device busy

I tried a umount -f just to see if some process was hanging on to it for
some reason (even though it was freshly booted just yesterday and I
haven't used the cdrom at all since then) and I get:

borges[142]% sudo umount -f /dev/acd0
umount: unmount of /dev/acd0 failed: Invalid argument

I had a similar problem before with 5.0-RELEASE and someone suggested I
set my cdrom drive as a "slave" (I had it set as secondary master) which
worked fine for some strange reason.  Come to think of it, I haven't
used the cdrom drive at all since I did a clean install of 5.1-RELEASE
last month so, AFAIK, my drive hasn't worked at all with 5.1 but was
coerced into working on 5.0.

borges[143]% dmesg | grep acd0
acd0: CD-RW <CD-R/RW RW7083A> at ata1-slave WDMA2

I thought I would ask here as I rarely have luck with questions
pertaining to 5.x on freebsd-questions.  Any ideas?

TIA,
Scott