The last time I investigated wireless HDMI transmitter solutions a couple of years ago they only transmitted over line-of-sight and cost well over £500. Fortunately things have progressed and there are now a number of more affordable options available.
Whilst looking (unsuccessfully) for the Raspberry Pi last week on CPC Farnell's site I saw that they had a special offer on a generic no-name transmitter-receiver set for £120 so I picked one up. (This has subsequently returned back to its normal price of £156).
I really wasn't confident that it would work, given the lackluster performance of the first generation devices. You'll need to watch the video to see if my pessimism was warranted or unfounded.
UPDATE - 25/08/12 - You can now buy a self-powered HDMI splitter that works well with this device. Blog entry here.