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 the 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.
A company named Pluggable makes several USB to Ethernet adapter 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.
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 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 grabbed an IP address via DHCP and now is just a regular computer on our wired or wireless network.
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 the 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