Open Safari and enter the CUPS address: http://127.0.0.1:631/printers/. Click on the Classes button, and then on the Add Class button. Enter the information you want for Name, Location, and Description -- the Description box will be what shows up in the Print dialog. Click Continue when done. Choose the printers you want in the pool from the displayed list; use the Command key to select more than one printer. Click Continue.
You're done. In the regular print dialog box, your pool will show up with the name you used in the Description box. If you print a job to the pool, it will be handled by the printer that is not currently printing a job. You can print another job, and it will be taken up by the next printer in the pool. You can get as many printers printing at the same time as you want.
[robg adds: This worked in my testing ... until it came time to remove the test pool I created. The CUPS browser interface button for Delete Class didn't work, claiming I didn't have any authorization to use it. This may have been an issue with Camino, but I just solved it via the brute force method: In Terminal, cd /etc/cups, then sudo rm classes.conf. Finally, I had to restart CUPS to make the pool vanish from the Print dialog:
sudo /System/Library/StartupItems/PrintingServices/
PrintingServices restart
After the restart, the pool had vanished from the Print dialog.]

