The DVD forum has approved a new version of the triple-layer HD DVD disc standard. The new specification allows manufacturers to create HD DVD discs with a capacity of up to 51GB:
The new 51GB HD DVD ROM disc has a three-layer structure with each layer storing 17GB of data, which is an advancement in capacity over current ROM discs, which hold 15GB of data in each layer of a single-sided disc. Continued improvement in disc mastering technology has achieved further minimization in the recording pit, supporting a further boost in capacity to 17GB in single layer and a full 51GB on a single-sided triple-layer disc. Toshiba has confirmed the disc structure and its successful operation earlier this year.
Unfortunately it's not clear if tri-layer HD DVDs will be readable by current HD DVD hardware.