This old thread deserves a "bump" because it shows how frail forecasts can be. Famous last words: "It'll never happen."
At the recent CES, LG introduced a 55" 4k UDTV that is only 4mm thick. When offered for sale later this year, no doubt it will be expensive. But, by 2013, one can bet other companies will offer similar gear and the price will fall.
JVC has just introduced a compact 4k videocam, which will go on sale for $5 in March. Before the end of 2012, one can bet there will be other contenders.
Admittedly, most 4k video will be limited to various commercial theater or big scale PR and ad displays for quite a while. 140mbps broadband will cost a lot. Existing movies, upscaled to 4k, won't look any different, especially beyond a certain distance or on a small screen. Premium sports content, at 4k, may be a leading edge, albeit pricey at first.
However, even in a world limited to 1920x1080 screens or 15mbps streaming, 4k still has value. Still photographers are able to crop large pictures and retain full image quality. It is a staple tool of the trade. This has never been easy in video, since any cropping of the video frame entails instant resolution loss. With 4k video, on the other hand, one can crop to 25% of the frame and still have the equivalent of full HD. This would be extremely handy for sports, action, or performance video, since it would allow you to shoot at wide angle with a stable camera, but then be able to "cut" to a detail or key piece of action without panning or fiddling with focus and zoom. Any DIY videographer should instantly perceive the advantages.