I know it's a bit premature, but in anticipation of the new experiment I picked up an XBox360 controller while I was at Target today for $40 (there was a $30 variety as well).
You see, character movement in a 3D world such as in WoW is ideally suited for a game pad with a pair of analog sticks. One can be used to control your "look at" vector, while the other can be used to control your "direction" vector. This allows for all combinations of movement and glancing, including strafing, gliding, and plain old-fashioned sprinting. As well, the analog functionality of a stick means you can control run/walk with a range of values between 0.0 and 1.0 rather than having to toggle run/walk settings.
Finally, a game pad encourages a much more action-based experience, as people are used to running, jumping, climbing, swimming, and swinging a weapon while using a game pad. These philosophies seem much less common in PC-style RPG's, which are more accustomed to auto-attack and wait scenarios.
For convenience the XNA Framework comes with a namespace called Microsoft.Xna.Framework.Input which has classes designed to control your computer/XBox's Keyboard, Mouse, and XBox360 Game controller.
Although we will primarily be using the Keyboard/Mouse classes for chat, UI commands, selection, and Object/NPC Interaction, we can use the GamePad classes for character control at the very least, if not for combat and some object/NPC interaction.
For those people that do not want/cannot afford to purchase an XBox360 GamePad, there will be Keyboard/Mouse bindings which will do all the same things we will be doing on the GamePad, though you will not have access to the force-feedback effects granted by the analog sticks.