Ive seen alot of people waiting for the Raspberry Pi, same here! it seems like a pretty sweet solution.
I have a somewhat more expensive alternative, but with built in wifi, android os and the size is smaller.
FXI Cstick Cotton Candy
With Android 4.0 plex would be a easy fix? or? Problem might be making a real client, more like the pc client, not like the one already on android market.
Software: Android 4.0 Ice Cream Sandwich
Ubuntu Linux for ARM
CPU and Memory: ARM Cortex A9@1.2GHz
Quad Core ARM Mali-400MP Graphics Processing Unit
Up to 64GB memory local storage (microSD)
Media support and Connectivity:
480p/720p/1080p decode of MPEG4-SP/H.263/H.264 AVC/MPEG-2/VC1
MP3, AAC, AAC+, Real Audio
JPG, GIF, BMP, PNG
Additional video, audio and image formats can be supported through 3rd party codecs
USB 2.0 male connector for power and connection to devices that supports USB mass storage
HDMI 1.3 Connector with audio
Bluetooth 2.1 + EDR