The first mainstream USB monitors were first presented at the beginning of 2007 at Cebit in Germany. The Syncmaster Ubisync 2040UX was produced by Samsung and allowed up to six monitors to be connected to the same computer with an additional two if you had a video card.
Fast forward to 2011 and USB monitors haven't taken over the world as we'd expected. The fact that they were limited by bandwidth (480Mbps) explains partly why that was the case and it is likely that the market was not ready for USB monitors.
Still, Samsung did produce a number of very affordable USB monitors (22-inch full HD with touch control) as did LG, Asus and Lenovo (we use two L2230X as our main monitors) but they have all been cleared out possibly because of lack of demand.
But things might change soon. Asus demoed a 24-inch LED USB powered monitor back in June at Computex and the big difference is USB 3.0.
This allows the monitor to use only one port to transmit both power and data; there's unfortunately no daisy chaining but that should not be a problem if you have enough USB ports (ed: might be an issue on Ultrabooks though).
Let's hope that 2012 will see more mainstream manufacturers dare to launch different products. One can bet that Apple will almost certainly launch a monitor powered ONLY by Thunderbolt.