After a number of years in the desolate wasteland that is 1080P, we are finally at a convergence of the television and monitor markets with 4K televisions. Based on the ability of Seiki to sell a 4K 39" panel for less the $500, it's likely that 2014 will usher in a series of relatively-inexpensive monitors delivering this resolution. Similar 1080p panels are selling for $300, and since the manufacturing isn't significantly more difficult, it's likely that in 12-18 months that pricepoint will be reached for 4K monitors as well.
Worth noting - the Seiki does all of this while including a remote, tuners, and multiple connectors unnecessary for strictly computer-use monitor. It's likely the costs (and prices) could drop even more in that sort of an implementation given enough volume. (Since the TV market is much bigger it may still make sense to make a one-size-fits-all model)
I purchased the 4K 39" Seiki TV about two weeks ago for use as a monitor. The 30Hz refresh rate is lower than I would like, but for software dev, still images, and watching the tiny amount of 4K video content it is completely fine. 39" is a little bit larger than I would have normally considered, but it offers a nice amount of screen real estate (less than 2 30" monitors though) and slightly higher dot pitch than a standard 2560x1600 30" display.
tl;dr It's completely worthwhile to get a 4K display now at this price.