I second the Ikea Dave because that's what I use. It's the perfect height for a typical couch and is adjustable too. If you pair it with a half-length keyboard, you will have room for a mouse. Personally, I use an Apple Bluetooth keyboard and a Logitech Performance MX mouse.
You could use a Microsoft Arc or their Mobile Keyboard 6000 if you prefer those key layouts instead. Personally, I love the Apple keyboard cause it looks great in the living room. The only drawback is that it has the FN key on the lower-left corner, and then CTRL is just to the right of it, but you get used to it. You can use a keymapper to make the Option into a Windows key and Command into Alt. My HTPC gaming rig is a dual-boot Hackintosh/Windows 7 machine, so it's good to have this sort of keyboard.