Comet C/2010 G2 Hill was picked up in the STEREO H1A imager by Jiangao Ruan and identified by Wen-Tao Hsu and Comet Al. The comet had been reported to be in outburst on the 3rd.
For those interested, I attempted to measure the intensity of C/2010 G2 Hill using images from the STEREO spacecraft, the comet came into view in the STEREO H1a instrument on 4 Jan, but was not in such a good position for attempted astrometry until the 5th.
My astrometry of C/2010 G2 Hill from 5-Jan to 7-Jan yields a magnitude of 12.1 for all days.
It's pretty hard to get decent values as the comet is very close to brighter stars and right close to the limit of detection (mag 13). I also can't work out accurately the 95% confidence interval for these values (but its on the order of +/- 1).
The magnitudes were generated by measuring the intensities of the comet and nearby reference stars in ImageJ then generating a standard curve from the reference stars in Graphpad prism, and interpolating the intensity of the comet from this curve (yes, I know this is rubbish, but it's what I have to work with). Reference stars are
TYC 660-105-1 8.45
TYC 660-293-1 9.81
TYC 660-877-1 10.93
TYC 660-48-1 11.22
TYC 660-579-1 11.8
TYC 660-944-1 12.57
Animation of C/2010 G2 Hill on the 6th, a small tail may just be seen. the image has been inverted from B7W to make seeing the comet clearer (uses images above to see location).