In the HDMI protocol it is called CEC, where by a device connected to another device via HDMI, can control certain functions on that device. For example, a TV can control the volume on a Audio/Video receiver connected to it via HDMI. So you use the TV's volume controls to control the AV Receiver. Also if you power on the TV, it will power on the AV Receiver for you, to make sure the sound can be output to it.
So much like that, the Pi support CEC out of the box, unlike most GPU cards for computers. So it can be controlled via the TV. As long as you switch the TV to the HDMI port connected to the raspberry pi, and you have XBMC installed with teh CEC plugin, it works out of the box. RaspBMC should already support this.
I know this works, because I did it for my parents, and was going to buy extra remote and receiver, when I figured oh wait the TV remote control works just fine. Parents are also very happy with that.