What have you tried so far in terms of troubleshooting this issue? Are you plugging the iPod into a high powered USB 2.0 port on the back of your Mac? Have you tried a different USB cable?
What happens if you try to reset the device with it still connected to the Mac?
Your profile says iOS 3.1.2, which only applies to the iPod Touch or iPhone. Are you sure you actually have an iPod nano?
Have you carefully worked through each and every single suggestion in this Apple support document?
I actully solved the problem... i restored my ipod classic, then while i rebooted my computed, the ipod mounted and showed up inside itunes, then i had to check the "enable disk use" button, then unmount my ipod inside itunes.... now all is right and my ipod shows up in itunes whenever i connect my ipod to my computer. seems for some reason, having the enable disk use checked was the key.