If I understand that article correctly, installing AV on the Mac (despite the fact that Mac AV does nothing for Leopard, Snow Leopard, or Lion) would prevent Flashback so it's okay to spend $$ every year for mostly Windows virus definitions for that ONE vulnerability?

Security is always a trade-off between risk and utility. AV on the Mac, just like AV on Windows, slows down the machine, often interferes with the running of legitimate apps, and sometime destabilizes the computer.

I would prefer to keep my money and stay in the safe parts of the internet. When the threat becomes one where almost every web site is a potential threat I may reconsider my position.