If I were Mr. Canon Inc., then I would ensure that all 5D2's were sold before the 6D arrives in-store, then I'd add the 5D2 to the DISCONTINUED list (just as they did with the EF 24-70mm mk I even before the Mark II was available).
This way, will guarantee more sales of the new 6D. It will also protect the residual values of 5D2's, especially for those who will ultimately trade-up to a 5D3 -> this is what Mercedes-Benz has been doing for decades to allow existing users a better price to trade-up from, whilst simultaneously protecting the brand and creating desire from aspirational buyers. In marketing terms its called a Win-Win scenario.
Canon are presently discounting the 5D2 to eliminate inventories prior to the arrival of new 6D models in early-December. It's a once-off. It will generate goodwill, whilst not really affecting the second-hand market (note that they did not price the last of the new bodies at or below where they're trading at in the 'used' market -> that type of deep-discounting would just piss people off).
By the way, to all of the voters who voted for Option #1 - the great sell-off and slide in 5D2 prices - a lot of CR bloggers said the same about the EF 24-70mm standard zoom lens earlier this year, and since it has been discontinued, prices have moved only one-way....UP. There may be lots of used 5D2 bodies floating around the US, but once Canon stop selling them, then they will become scarcer and scarcer as time goes by. And for those people who think that there is some magical equilibrium price of a used 5D2 of say for arguments sake $1000 based on a naive understanding of economics (Supply & Demand being determined by both Quantity and Price), well that is Elementary ECON-101 in the two-dimensional world. The real world is multi dimensional so there are multiple equilibria: one based on the 5D2 being discontinued, one not, one based on the 6D having lower IQ, another based on more crop-body owners trading up to FF, and so on and so on).
It is not a zero-sum game, the pool of Canon FF body owners will likely increase with the advent of the 6D. My guess, is that Canon know this (Demand > Supply overall) already having sold more than 30 million DSLR's and having sold their 80th Millionth lens this summer. Canon know precisely what they're doing...at all times.