Which version of USB do iPod Nanos have?
Apple's tech spec says it works with a Mac having 2 or 3, but USB is backward compatible so that doesn't help...
All iPhones and iPods with the Lightning connector use USB 2.0. From Wikipedia:
The Lightning connector … provides an interface to USB 2.0 (for data and charging)
Also, if you look the USB end of a Lightning cable, it doesn't have the additional pins for USB 3.0.
There is actually no point to having USB 3.0, since the flash memory in iPods and iPhones is not fast enough to benefit from its speed.
The current (7th) generation of iPod Nano (I'm assuming your referring to the 7th) utilizes a Lightning port. This port in turn utilizes a Lightening to USB 2.0 cable, that in turn plugs into the appropriate USB 3.0 (or earlier) port on your computer. In other words, it will plug into a USB 3.0 port even though it's only a USB 2.0 cable.
Here is a link to the actual specification
Per the Apple specification:
"Mac computer with USB 2.0 or 3.0 port, OS X v10.6.8 or later, and iTunes 10.7 or later"