I have to give the Portal team some epic geek credit (and some to myself for actually trying this out) – if you record the signal played on the radio when you find the “Final Transmission” achievement location, you’ll have a SSTV signal which you can decode into an image of the companion cube on the moon. So, I did. This screenshot is the decoded result.
Update: I’ve just uploaded the transmission sound clip, so you can hear what the transmission actually sounds like: Portal – Final Transmission.m4a
Williamo's Blog 