The DVD+RW Alliance has announced that version 1.0 of the Double Layer DVD+RW specifications have been finalized and are ready to be distributed to licensees. Here's part of their press release:
The DVD+RW Alliance, a voluntary association of industry-leading personal computer, optical storage and consumer electronics manufacturers that support the Plus format for DVD recording, has announced the release of version 1.0 specifications for 2.4X rewriting on Double Layer DVD+RW media by HP, Mitsubishi Kagaku Media/Verbatim, Royal Philips Electronics, Ricoh, Sony Corporation, Thomson Inc. and Yamaha.

DVD+RW media and writers are designed to rewrite up to a full 8.5 GB DVD+RW. The new 2.4X DVD+RW Double Layer discs will be based on new phase-change materials, which enable writing on both the semi-transparent layer 0 and the more reflective layer 1.

Due to its special characteristics, the new DVD+RW Double Layer media will only be read and write compatible with new specially designed recorders and read out devices. The read out reflectivity on both layers of the new media is very low when compared with traditional single layer DVD+RW media. Red laser reflectivity is specified between 5 and 10 percent for the new media and 15 to 20 percent for traditional single layer media.
The DVD+RW Alliance has not announced when Double Layer DVD+RW media will be available, but now that the specifications have been completed, it will only be a matter of time. If you'd like to read more, the entire press release can be found here.