To scan the letter in my last post I used our new Brother 5440CN multifunction machine. Not only was it easy to setup for printing, it was a piece of cake to get scanning working!
From the 5440CN driver page I grabbed the Mac scanner driver. On installation it wanted to know where my machine was (clicking 'Browse...' brought it up, presumably using Bonjour again), and I was able to tell the scanner a name to use for my computer ('Chris').
After a reboot (huh?) I put the letter on the scan bed and hit the 'Scan' button. The scanner screen prompted me to 'Select' and hitting the down-arrow led me to 'File'. I selected that and my name came up -- nice! So I selected that and it prompted me to hit the start button. I did, and a couple seconds later a 'Scanning...' progress bar popped up on my powerbook. When the scanning stopped I had a fresh new file in my ~/Pictures directory with the color scan. That's how this stuff should work!
(FWIW, you can also pull the scan from your computer if you want, using the Brother Control Center software.)
So if anyone needs a flatbed SCSI scanner (including a dedicated PCI "SCSI" card), let me know.