Sign up to AV/Hexus forums to get free shipping on scan, only takes a few seconds. Scroll down a tiny bit on the link to the 'Super Savers' where you will find this deal under Power Supplies, click the buy icon to get the Today Only price.
Anyway I think this is a pretty good deal for those looking to build high end rigs, it's not the cheapest but you get 7 years warranty and modular PSUs are always easier to deal with.
This particular PSU seems to be retailing for £130-£140 elsewhere so pretty good deal I'd say, may have to get myself one :)
Here is a review for those interested http://www.hardwaresecrets.com/article/Corsair-HX850W-Power-Supply-Review/741/1
which seems to speak quite positively of this power supply.
# Guaranteed to sustain its full rated wattage at an ambient temperature of 50°C
# Up to 90% energy-efficiency under real-world load conditions*
# Single +12V rail design providing up to 70A
# Multi-GPU ready
# 105°C solid state capacitors
# Active Power Factor Correction (PFC) with PF value of 0.99
# Supports ATX12V 2.3 and EPS12V 2.91 standards. Backwards compatible with ATX12V 2.01
# Auto switching circuitry for universal AC input from 90-264V
# Over Current/Voltage/Power Protection, Under Voltage Protection and Short Circuit Protection provide complete component safety
# Dimensions: 150mm(W) x 86mm(H) x 180mm(L)
# MTBF: 100,000 hours
# Safety Approvals: UL, CUL, CE, CB, FCC Class B, TÜV, CCC, C-tick