Today's Arduino lesson is that not all Arduino Nano v3 boards are the same (and this is probably true for other Arduino models when purchased from different sources). Let's skip over how long it took me to discover that, m'kay? I discovered this by looking in System Information, going to System Report, and looking at what was connected to USB. Get drivers for that, whatever it is.
My first Nano required PL2303 serial drivers. My second Nano needs the more standard FTDI libraries instead. Sparkfun has FTDI installation directions, and those drivers can also be downloaded directly from FTDI.
And just like that, sprinkling a little driver magic over it, this Arduino is now happy to talk to its IDE.