Christmas 2007 for me.  I always wait a year.  The price will most likely go down some by then and the games will be getting much better by then.  It always takes a year for the games to really start utilizing a new peice of hardware.
I'm not so sure that 2007 will be early enough for a price drop on the PS3...and even if they did drop it $100 to a $500 price, by then 360 will be down to about $300 and Wii to $150 or so.
I agree that the game quality will likely be on the upswing after the console's been out a year, but imo that still won't be good enough for me...I'm still thinking at least 2010 before I get one.