I just deattached/attached and things started working correctly again. Powerblade said that he had just increased the cache size to 200 per computer. I'm wondering if maybe those 3 machines asked for Nvidia work, the project said it didn't have any, and then the BOINC scheduler screwed up and started asking for ATI wu.
I've never had a problem with 6.12.34 so this was something new that just happened.