So, after installing an nVidia GTX 650TI a while back, I put up with tearing in pretty much all applications because it was easy to install and get going.
But eventually I got sick of it and tried to fix it. After trying about every fix I could find, I jumped to a different distro (from Xubuntu 12.10 to Mint 15) in the hope that might have some effect. It didn't, but I re-tried a fix I thought I'd already tried, and found it eliminated the tearing 100%.
The fix is to add this to the /etc/environment file:
This is using the nVidia proprietary driver 304.88. Hope to confirm on other Xubuntu systems whether this fix also works.
Update 20 June 2013:
Unfortunately, the fix only seems to work in Mint/Cinnamon. Title updated to reflect this.