PocketDock combo is a device you attach to your iPod that gives it a firewire and USB out. With it, you can connect to a USB DAC. There are a few high-end USB dacs on the market, but I can't think of the names. Here's the url to the product page...
http://www.sendstation.com/us/products/pd_combo.html
http://www.sendstation.com/us/products/pd_combo.html