The M9 will surely be an excellent camera and a welcome addition. However, the M8 is no slouch and it produces images/prints that defy its 10 mpix cropped sensor resolution. I think that most current M8 owners wanting to upgrade to the M9 will only sell their M8s if they can get enough money out of them - I don't think they will give them away for ridiculous low prices. I also expect the used M8 prices to be relative to the price of the M9 such that a high priced M9 will help hold value for the M8. If the M9 price comes in around the current M8.2 price, then I think M8 prices will tumble below $2k. If the M9 comes in at $7k or $8, then I think M8 prices will likely stay above $2k for a few months.
Of course this is just me thinking out loud and contemplating whether to sell my M8 for an M9 or keep it for use with longer focal lengths and as backup. It will all depend on the M9 price and its affect on the used market. Also, I expect used lens prices to go up due to demand because I think there have been a good number of people waiting for the M9 (full frame, no IR filters). Or who knows, sell the M system and get an S2 along with an X1.
I am shocked, it now appears that, as of 9/9, Leica will have three cameras I want. That didn't seem possible just a couple months ago. Bravo Leica. Its too bad I don't have enough cash for all three.
Mark