I think road tax may play a part in getting people away from bigger engined cars, but ultimately it wont play too much with prices. I tend to change cars every 2 years or so, for me, I'd expect to get £500 off if I was buying a bigger car nearer to the time, just to offset the extra couple of hundred quid per year it will cost to tax from 2009.
Bear in mind the price of decent capacity diesel engined cars may go up due to the more attractive mpg and tax band, thus the gap will be wider still, in which case it may push people back into big engined cars, if that makes sense?