Behodar: If I get time tonight I'll connect my TV to the second port on my 560 and see whether I can use both displays.

I knew there was something I meant to do... I did test it but forgot to post back. It didn't work; no video on the second port (even after a reboot) :(

Update: Both ports work under 10.8 (GTX 560 Ti) using the Apple-supplied drivers.