A community memory over at [H]ardforum has created an input board and LCD panel controller that supports a few monitors out there that allows you to upgrade your 4K monitor to 120Hz. Or, you can play at 1080p 240Hz or even 540p in 480Hz mode. If you are not keen on installing the kit in your monitor on your own, he also sells a 28" fully assembled monitor with the custom board already installed. You can read the mini review here: http://www.blurbusters.com/4k-120hz-with-bonus-240hz-and-480hz-modes/ And if you would like to order your own and be one of the first people to own a 4k 120Hz monitor, then you can purchase one here: http://www.zisworks.com/shop.html
Dang, that's pretty cool. I wonder if the 4K120 monitors will be near the same price as this during launch. The only downside is having to upgrade GPUs. I'm not sure 1080Ti SLI would be enough due to scaling for most modern games.
The ASUS one has a few more features than this due to the new GSync module, but it is rumored to be $1500 or more when it goes on sale.