Two monitors side-by-side at the office for the past 10 years (various resolutions depending on time period), but at home I've just hooked up the new Asus PB287Q 28-inch 4k UHD monitor to my Mac Pro.
Asus lists the resolution at 3840 x 2160 pixels (so-called 4k standard) but when I hooked it up via DisplayPort 1.2 to get 60Hz refresh rate (and made other calibrations), the "About This Mac / Displays" pane states 5120 x 2880 pixels (for 14.75-million total).
Thankfully the most recent OS X updated to 10.9.3 allows for scaling of most text and icons so my aging eyes don't have to squint too often; sitting 60 cm from the screen, the effect is certainly retina-like. Only downside… the PB287 is TN panel rather than an IPS (those 4k monitors are in the $3,000+ range). Maybe in a couple more years?
Senior Associate, Chernoff Thompson Architects
ArchiCAD 16 (firm uses Revit)
Mac OS X 10.10 on Mac Pro (2013)
3.5 GHz 6-core Intel Xeon w/64 GB RAM & Dual AMD FirePro D700 w/6 GB Graphics
1 TB SSD w/20 TB RAID 1
Asus PB287Q 4k UHD 28-inch monitor (3840x2160)