uh, any keyboard that has wireless functionality would work fine... most wireless receivers have a range of like 30 feet... I know my wireless mouse (g7 logitech) works through 2 walls and like 20 feet away.... so you should be ok once you find a keyboard that you like aesthetically or ergonomically
plug and play in both vista and xp and it's only $30 bucks. the quality is good and best of all it has a trackball so you don't have to worry about a mouse (which i really like). The range i'm using it is at about 12 ft with no problems.
if you have more money the logitech DiNovo edge is very nice and has a laptop style touchpad, it's bluetooth and i haven't used it but it also has good reviews.
It has a built in touch pad (upper right) that can be switched to a joystick type control for the mouse. Plus it comes with an actual mouse if you wanna have that. better value then the diNovo edge IMO.
I've been using the above mentioned Moneual keyboard for my HTPC for about 8 months. It has very good range and battery life. I love the built in track-ball and the small size of the keyboard. The only cons I can think of are that the top application keys are not re-assignable and sometimes when I upgrade an unrelated driver (like the video driver for example) the keyboard doesn't connect upon reboot. In that case, I have to unplug, then plug back in the USB dongle to re-establish a connection. This is a rare occurence though.
For a living room HTPC you want something that makes an impression. I've just bought the Logitech Dinovo Edge. Bluetooth, touch pad (could be a little more sensitive) and nice little coffee table recharger stand. Press the button and function keys light up in orange for the my media centre controls.
I'm very happy with it although for extended mouseplay I would attach one.