You can also try
Mini-Box and
Logic Supply. Logic Supply has some cool cases including the Chenbro units (great for WHS). Mini-Box manufactures the picoPSU and various slim, aluminum enclosures.
Zotac makes an $80 mini ITX board that comes with a WiFi adapter. Don't know if the integrated video is up to snuff on playing back stuff, but they also make a $150 mini ITX board using the GeForce 9300 chipset which should be fine.
For a remote, how about an RF instead of IR? That way you won't have a receiver dongle sticking out the front of your cabinet. I've had a chance to use the
Gyration Air Music Remote. It uses RF and has a USB dongle that does not need line-of-sight and indeed can function from a room away (or more). The base function is that of an MCE remote, complete with "green button." Additionally, it has two functions over normal MCE remotes (besides RF wireless). It has the Gyration mouse built-in. I've used a Logitech MX Air and the Air Music Remote is much easier and precise to use as a mouse. The final function is an LCD screen on the remote. Basically you can use your computer "headless" meaning your big-screen TV doesn't even have to be on. This means if you are just going to play some music and not video, you can use the LCD on the screen to browse your media collection without having to turn on your TV. You can even do this from another room in your house thanks to the RF instead of IR. It is interesting that this gets Media Player to play in the background with no console. I tested with Vista 64 Ultimate.