The Popcorn Hour C-200 seems to be the top of the pile at the moment, but given it’s price tag (c. Â£280 before HDD, Blu-Ray etc.), I’m inclined to agree with vroom and suggest a PC.
For a media-player PC the CPU and mobo can be fairly old as long as they support the graphics card and sound card that you want. You’ll probably want a graphics card with HDMI out (1080p) and a soundcard with digital coaxial/optical pass-through if you have a surround amp and/or component out if you have a PC surround system. Other than that, a basic HDD, wired/wireless LAN depending on location and a wireless optical mouse to control it from your armchair. One other consideration is fan-noise, so you might want to invest in a moderm PSU with low-noise fan.
I haven’t fully spec’d my media center PC yet, because I’ll want it to do video processing as well, which means I’ll want a better CPU and 64-bit architecture… a job would be handy first, though