yep, at distances that would allow an immersive experience for increasingly popular 80/84/90-inch panels, 4K is great to have and allows a virtually invisible pixel structure. Since the HDTV market has become particularly stagnant I wouldn't be surprised if content came sooner rather than later.
I haven't seen the 84" Sony yet, but I'm pretty familiar with 4K up-conversion via the JVC DLA-X70 on display at work. The 4K "e-shift" (3840x2160) looks great on our 115" screen and makes Blu-Ray content look pretty amazing.
ht Panasonic TC-P60ZT60, Monitor Audio: Silver RX6, RX Centre, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT
lr Panasonic 50ST60, Sony BDP-S5100