First off, it will be impossible to generalise.. but in some specific instances the price will absolutely have to drop.
Take the BMW 520d.
BWM have promised to pass on the full savings of VRT (with a provisio that they may make some items that were previously extras, available as standard specification but with the value of them fully traceable back to the VRT saving).
So, on standard 520d there is a saving of 9500. Also, it will now be taxed at 150 instead of 590.
So, a 08 reg 2nd hand car being sold on the first of July, will not only have to compete with a new car being available for 9.5K cheaper, but also will have a 440 eur a year extra tax cost for it's lifetime.
Therefore it is a given that this model will have a serious adjustment in it's residual values. Given the euro-sterling rates, it becomes a no-brainer to bring in a 520d from the UK (even at today's rate it's cost effective - not to mind when the vrt drops from 30 to 16 per cent) so this will have a very large impact on this particular model.
The opposite also applies of course - some vehicles - e.g. the Land Rover Freelander, go from 530 a year in tax to 2000 a year, with the VRT going from 25% to 36%. So - residuals on the existing Land Rover pre July will remain strong.
So the short answer - no generalisation possible - down to model by model.
I honestly believe there will be a change to this before too long - probably something like people with cars pre- July 1st will be given the option of using the new annual tax structure.