I found this issue last night and a moderator got in touch with me via email to help fix the problem. As soon as I got the email from the moderator, there was a suboption for the Nano but it won't recognize the S/N. Keep on pressing ok or accept even if it gives you the error message, after the second or third error message, the page will ask you for the product number (MPN). Get the box where the Nano came in and look at the bottom of it. You will have 2 numbers: your S/N then your MPN # that starts off with M. In Nano's case it's "MA107LL/A" without the "". Click on okay and it will say again that it doesn't recognize the serial number but it will give you an option to proceed with the registration anyway.
The weird thing is that if you go to
http://www.apple.com/support/ ... on the middle right hand area of the page under "About Your Apple Product" when you put in your Nano's S/N, it recognizes it and tells you how many days of warranty you still have left.
The moderator that I spoke to is aware of the problem and the tech guys are working on fixing the issue.