HDMI 2.0 just got announced, featuring official support for 4K 60Hz, and 18 gigabits/second of bandwidth. Also announced on engadget and reddit.
4K 120Hz using 4:2:2 chroma at 8 bits per channel is only 15.9 gigabits/second, plus a little bit for reduced blanking intervals. Since this is less than 18 gigabits/second, this theoretically makes 120Hz 4K potentially possible over HDMI 2.0, assuming 8b/10b encoding can be bypassed.
People are unofficially doing 1080p @ 120Hz over HDMI 1.3a since the 340Mhz dotclock of the older standard is sufficient to permit this mode.