On the inability to support hardware that nobody makes any more
Windows Vista will not have support for really old DVD drives. (The information below was kindly provided to me by the optical storage driver team.)
When PC DVD drives first came out in 1998, the drives themselves did not have support for region codes but instead relied on (and in fact the DVD specification required) the operating system to enforce region coding, with the further understanding that starting on January 1, 2000 all newly-manufactured drives would support region coding in hardware rather than relying on software enforcement. For the purpose of this discussion, I will call the two types of drives "old" (manufactured before 2000) and "new" (manufactured on or after January 1, 2000).
It is that software enforcement that is going away. Turns out that the enforcement of region coding in software had its own problems:
* It was impossible for third-parties to compile their own CDROM.SYS from the source code in the DDK because the region code enforcement code was not included in the DDK.
* The region code enforcement code would sometimes mistake a new drive for an old one, resulting in customers unable to play DVDs. Even worse, the driver test team could not reproduce the problem reliably, and the problem went away entirely once a debugger was attached to the system.
* The code to support the older drives is complex, and the drives that the optical storage team purchased prior to January 1, 2000 are dead or dying. Consequently, testing the code that provides support for old drives has become increasingly difficult, and when the last old drive finally gives up the ghost, testing will become impossible altogether.
These were among the considerations which contributed to the decision to stop supporting these old drives.
What does this mean for you? Almost certainly, the answer is "absolutely nothing".
First, there is no change to the way data is read from DVD drives, so data discs will still work the same way as they do today. Second, all new DVD drives will continue to run as they did before; the only change is that the risk of mis-identification as an old drive has been removed. Only if you have an old drive will you notice anything different, namely that encrypted/regionalized DVD movies will no longer play. And since the average drive lifetime is only three years, the number of such old drives that are still working is vanishingly small. Not even the optical drive test team can manage to keep their old drives alive that long.