The eagerly awaited Acer Aspire Revo is available to Preorder from Play for £149.99 (its due out on 18th May).
This appears to be the best price around.
Yes I know that 8GB SSD is a bit tight - if you want more storage then either pay through the nose for the models further up the range, open it up (one screw) and swap out the 2.5" hard drive or use an external one.
This tiny PC is capable of playing 1080p video and it is virtually silent so therefore it makes an ideal second PC to attach to the lounge TV via HDMI. It should work well with with Boxee for Linux.
For more info on this machine, look herehttp://hothardware.com/Articles/Acer-Aspire-Revo-SFF-NVIDIA-Ion-PC/?page=1
UPDATE - Also available at Amazon for the same price - Thanks elmo61 (post 41)