First, I'm fairly certain this voids your warrenty. Second, I'm not responsible for any damage or injury to you, your Roku, or anyone or anything else that may result from attempting anything described below.
I have always used the wired interface for network connectivity and have no need for the built in Wifi. I have also always wanted to upgrade to an XR or XD/S for the USB port to access local media. I did some poking around and googling and it turns out the Wifi card is on the Roku USB. I opened up my Roku and removed the wifi daughter board. It is connected via a 6 pin header on the mainboard. Using my multimeter I quickly discovered the apparent pinout.
1. 5V+ 2. NC
3. D- 4. GND
5. D+ 6. GND
I made a quick trip to Frys and picked up a "2 USB Ports to 2x5 w/ Bracket".
The pinout for this is basically correct to hook it up to the Roku USB header.
1. RED 5V+
2. WHITE D-
3. GREEN D+
4/5. BLACK GND
I removed the two black wires from the connector (use a little pin to pull up the tab and slide out the metal sleeve). Then I simply plugged in the connector, the red wire into pin 1, the white wire into pin 3, and the green wire into pin 5, the two black wires get plugged into 4 and 6.
I now have a wired network only XR for $5 more dollars and a little hacking.
While at Frys I also picked up a Populated RS232 Module to hook up to the TTL header on the board. More on that later