Hmmm, this probably tackles some of what I said above..
From the Pioneer site:
Writing with 2X DVD-R Media Beginning with the launch of the DVR-A03/103 DVD-R/RW drive, Pioneer has included a feature allowing 2X recording of DVD-R. This feature has been brought about by Pioneer's own engineering efforts. Within each drive are media specific optimizations called 'write strategies'. These write strategies are tailor made for specific media brands, making 2X DVD-R recording possible for those submitted to Pioneer.
Currently, the DVD Forum's specification for DVD-R recording is at 1X speed. Pioneer is able to base its 2X DVD-R recording feature on the DVD Forum's specification, making it possible to include this benefit to its customers.
Before Pioneer can support a candidate "For General" DVD-R media brand with 2X recording, the following steps must be completed. First, the media must pass the DVD Forum's verification tests for 1X DVD-R. After clearing these tests, Pioneer, with the media maker's cooperation, fine tunes the drive's write strategy for 2X recording, and then confirms the media's performance with mass production DVD-R samples. Pioneer can only consider DVD-R media to be 2X compatible after it passes these tests.
This development system, coupled with a given media brand's release date, means that 2X DVD-R recording may not be possible with all media. Unlike high speed CD-R recording, 2X DVD-R recording requires optimization. Without optimization, the possibility exists that the recorded media will lack the desired playback quality. This system has been implemented in order to avoid customer dissatisfaction with the performance of DVD-R media that is deemed not compatible with 2X recording.
Pioneer will continue to contact various media manufacturers to increase the number of 2X record compatible DVD-R media.
As of March 20th, 2002, Pioneer has qualified the following DVD-R media brands for 2X recording on our DVR-103/A03 and DVR-104/A04 series models. This list will be updated as more 2X DVD-R media brands become available.