non aligned DMA transfer attempted

I'm having a bit of a problem getting my DVD drive(s) to work correctly. 
  I'm trying to transfer my DVD collection to my media server, but 
whenever I run vobcopy, /var/log/messages gets spammed with:

acd0: FAILURE - non aligned DMA transfer attempted
acd0: setting up DMA failed

I added a bit more information to the first message to see if I could 
figure out what was actually going on.  request->data was 0xd40e0c37, 
ch->dma.alignment was 2, and request->bytecount was 2048.

My first guess was that my DVD drive was starting to flake out, so I 
bought another one in the hopes that it would fix it, but that wasn't 
the problem.  The drives that I've tried are:

DVDR <NEC DVD RW ND-3500AG/2.FD> at ata0-slave UDMA33
DVDR <TSSTcorp CDDVDW SH-S223L/SB02> at ata6-master SATA150

The ata controllers are detected as:

atapci0: <nVidia nForce MCP55 UDMA133 controller> port 
0x1f0-0x1f7,0x3f6,0x170-0x177,0x376,0xec00-0xec0f at device 13.0 on pci0
ata0: <ATA channel 0> on atapci0
ata0: [ITHREAD]
ata1: <ATA channel 1> on atapci0
ata1: [ITHREAD]
atapci1: <nVidia nForce MCP55 SATA300 controller> port 
0x9f0-0x9f7,0xbf0-0xbf3,0x970-0x977,0xb70-0xb73,0xd800-0xd80f mem 
0xcfffd000-0xcfffdfff irq 20 at device 14.0 on pci0
atapci1: [ITHREAD]
ata2: <ATA channel 0> on atapci1
ata2: [ITHREAD]
ata3: <ATA channel 1> on atapci1
ata3: [ITHREAD]
atapci2: <nVidia nForce MCP55 SATA300 controller> port 
0x9e0-0x9e7,0xbe0-0xbe3,0x960-0x967,0xb60-0xb63,0xc400-0xc40f mem 
0xcfffc000-0xcfffcfff irq 21 at device 14.1 on pci0
atapci2: [ITHREAD]
ata4: <ATA channel 0> on atapci2
ata4: [ITHREAD]
ata5: <ATA channel 1> on atapci2
ata5: [ITHREAD]
atapci3: <nVidia nForce MCP55 SATA300 controller> port 
0xc000-0xc007,0xbc00-0xbc03,0xb800-0xb807,0xb400-0xb403,0xb000-0xb00f 
mem 0xcfffb000-0xcfffbfff irq 20 at device 14.2 on pci0
atapci3: [ITHREAD]
ata6: <ATA channel 0> on atapci3
ata6: [ITHREAD]
ata7: <ATA channel 1> on atapci3
ata7: [ITHREAD]

Setting hw.ata.ata_dma and hw.ata.atapi_dma to 0 doesn't help any. 
General drive usage (browsing through the contents of the DVD, watching 
a movie with VLC) doesn't give the DMA errors.  I'm currently using 
8.0-BETA3 r196774 i386.  Any suggestions?

Ryan