It appears that even hackers don't take too kindly to Vista. They are pirating Windows XP at about double the rate of Vista.
One way to look at this is that people don't even want Vista if it's free.
Another way to look at it (the way we look at it) is that Vista is much harder to pirate, while XP practically gives itself away on street corners.
This could change when Vista SP1 releases, which will change how it deals with "non-genuine" copies of the OS. It should get a lot more relaxed.