Found this over at XDA - looks like ASUS know about your serial number problems and are working on fixing it soon

Good Day,

For those users who are suffering from the "Serial Number Not Found" problem we are currently trying to replicate the problem. In the meantime, there has been some success following these instructions.

Issue -
1. System will not auto or manual update to the latest firmware or image.
a. Go to Settings, Backup and Reset, start a Factory Data Reset. Please do this and after the reset let the unit remain active (settings, display, sleep, always on) for 15 minutes with a WiFi connection active so the system can sync with the ASUS servers. Hopefully the system will see the update, if not please go to settings, About Tablet, and then double tap on System Firmware Update please. If that does not work, please PM the serial number of your unit.


1/16/12 Update - We have replicated this problem and hopefully we will have a solution shortly.

1/17/12 Update - Thank you to the users I reached out to for their contact information. You should hear from our engineering or technical support staff shortly to verify a few settings and to try another OTA update sequence. For those I did not contact yet, we might expand the group for additional testing. I have read all the messages so please do not be upset if we have not contacted you yet. We are very close to finalizing a fix for this problem but first we will have to confirm your unit setup.

Official ASUS Thread - Serial Number Not Found - xda-developers