Microsoft seems to think that there are a lot of people that are going to be resistant to Windows 7 for one reason or another and have extended the availability of XP downgrades.
Originally, Microsoft was only planning on XP downgrades for six months following the launch of Windows 7. This has now been extended out to 18 months or the release of Windows 7's first service pack. Given the success of Windows 7 so far and the increase in quality that has been seen in the beta and RC versions, it should be quite some time before we see SP1 for Windows 7.
Why Microsoft has extended the availability of the downgrades isn't for certain. One thing that we can be certain of however is that this will continue to hurt the push towards native 64-bit applications. While the hardware base is out there, unless we can get to an overwhelming install base for 64-bit OS's, many application dev's aren't going to feel the need to develop 64-bit apps as much as they would without XP around.