I will take that bet BHR
Gran Turismo 5 will NOT sell 3m copies first week.
Europe and more importantly UK is Gran Turismo's biggest selling region. Things have changed a lot since ps2 generation. At the time of GT4 releasing there were 7.5m PS2's in UK and it was a game on a console that had no real competition. This time around for GT5 there are just 3m PS3's in UK, a drop of over 50%. Plus there are two other competitors which have both outsold it. In fact even 360 is nearly at sales of 2 to 1 over PS3 there. That will have a huge effect on sales this time around.
Plus Pro-logue was never a full price game. It started at £25 here, nearly half the price of a full titled game. Then in a matter of weeks it was at the £15 mark and below, which it is at today. Plus the game was bundled with most PS3's here for a period, it came with mine. I think they even ran a LBP GT5:p bundle for ages if i remember right.
If you look at UK pre-order sites charts your see it mirrors this chart pretty much.
If you look at those charts GT5 seems to be around the 8-10 mark. With even other racing games ahead of it i.e F1 2010.
This generation is full of games that sold less then their previous game in the last gen i.e GTA4. I expect GT5 will be another one of those games.