I just bought a new XBox 360 Premium yesterday (15-March-2008) and when I got it home I thought I'd make sure I had the newer Falcon 65 nanometer technology unit. I found an article which said shine a flashlight into the bottom and if you can see a copper heat pipe you have the old one, and it turned out I did! My box had a manufacturing date of 2006 so I was pretty miffed and took it back to the shop (Comet) and complained about them selling two year old stock. To their credit, they took it back even though I had opened it and they swapped it for the newer model. The cardboard box has a little flap in it so you can see the manufacturing date, and you want later than 9/2007, but an even easier way to see is on the outside of the box it says HDMI (yes even for the premium), and the power rating is 175 watt instead of 208 watt for the older model. This is also displayed on a sticker on the outside of the box.
Why should you care? Well the new one uses less power, is noticebly quieter and has the HDMI port off the Elite, which even if you don't need it, is going to make it easier to sell on eBay when you come to get rid.