Here is how I got my wife's Surface RT to work with a USB to Fast Ethernet Adapter.
I purchased a Belkin USB 2.0 Ethernet Adapter 10/100 MB from a Microsoft kiosk store.
The adapter is $24.95 from Microsoft and about the same on Amazon.
This adapter worked great with no drivers needed on my Surface Pro.
For some reason Microsoft doesn't want the Surface RT to be able to use wired Ethernet.
If I'm wrong, it would be fine with me if someone from Microsoft would correct my understanding of this problem or share their grand plan.
A company named Pluggable makes several USB to Ethernet adapters 10/100 & 10/100/1000.
This company has posted on their website that they were requested (demanded) by Microsoft to remove the drivers for their products that are compatible with the Surface RT.
Sounds crazy, but if you don't believe me, a quick Bing or Google search will get you the information.
Once I realized that the Surface RT could in fact use a USB to Ethernet adapter and just needed the correct drivers I started searching again.
Here is what I found:
The Belkin Adapter uses the AX88X72A chipset and that's what shows up in Device Manager for both the Surface Pro & Surface RT.
I believe this is the same chipset used in the Cisco USB 300M product. So the drivers below may work for this adapter as well.
An additional nice behavior is when the Ethernet adapter is being used, the wireless NIC is disabled automatically and becomes available again as soon as the adapter is removed. This has to be a power saver.
I found the correct files below and installed them as described below.
My Wife's Surface RT can now connect to our home network via wireless or wired.
Once the correct drivers were installed the Surface RT recognized the adapter immediately and has worked perfectly ever since.
Just in case the links are disabled; here are the names of the 3 files that are needed
AX88772.cat, AX88772.inf, & AX88772.sys.
As far as how to install the drivers, it's really like you would install any other drivers on any other Windows machine.
In Device Manager I right-clicked of the AX88x72A icon with the yellow ? (question mark),
chose update drivers, and browsed to the folder with the files.
Surface RT grabbed the files, it took about 30 seconds.
The Surface RT then grabbed an IP address via DHCP and now is just a regular computer on our wired or wireless networks.
I hope everybody gets a chance to grab the drivers before the links are disabled or removed.
Great product, shame on Microsoft to intentionally hobble this machine.
For Drivers
https://docs.google.com/file/d/0B6huq0bVEpNKYWNGNnJ3d3VNRzQ/edit?pli=1
How to install drivers
http://plugable.com/2012/11/16/usb-ethernet-microsoft-surface