This is an implementation of an idea on the Plex forums - that the Raspberry Pi could use a Plex client that had no interface, and was just designed to be operated using an iOS device or similar as a remote. Only the very barest bones functionality is here, but I hope that it is reasonably easy to extend.

sudo apt-get update && sudo apt-get upgrade
sudo wget https://raw.github.com/Hexxeh/rpi-update/master/rpi-update
sudo cp rpi-update /usr/local/bin/rpi-update
sudo chmod +x /usr/local/bin/rpi-update 
sudo rpi-update
cd /boot
sudo vim config.txt > to set arm_freq to 1000 and gpu_mem to 192
sudo reboot
sudo apt-get install avahi-daemon
sudo apt-get install python-pip
sudo pip install tornado
sudo pip install pexpect
sudo apt-get install python-avahi 
sudo aptitude install python-gobject

git clone https://github.com/megawubs/pyplex.git

Launch with

python pyplex [hdmi] &

Where [hdmi] is optional to make sure audio is going over hdmi, leaving it out will devault to the 3,5mm jack output. The & makes it run in the background.

Then 'Raspberry Plex' should appear as a player you can choose in your Plex client. Choose your media, and select this as the player to play it on. It should begin playing on your Raspberry Pi!

Stop with

python pyplex stop

To make it run in the background at boottime do crontab -e and add

@reboot python /path/to/pyplex

To control playback you can use the remote tab on your iDevice or android device. Currently the following commands are supported:

Play
Pauze
Fastforward
Fastbackward
Stop

In the root folder of pyplex do

tail -f pyplex.log

to see all logging information. Support can be found on the Plex forum

If you want to know more about the pyplex code and how it works take a look here