Simon Sharwood at The Register points to a paper in Nature Communications (and a more readable explanation) by a team from Swinburne University of Technology that may eventually allow for a petabyte on a single DVD platter.
They have found a way around Abbe's limit, which restricts the width of a light beam to be more than half its wavelength. They use two beams, each of which on its own is more than half a wavelength wide. One is round, and one is donut shaped and they overlap. Then, as with normal DVDs, they use a medium which contains a dye activated by the round beam. The secret is that the donut-shaped beam prevents the dye being activated. So the size of the written spot on the medium is the size of the hole in the donut, in their case only 9nm across. With 9nm dots it is in theory possible to get a petabyte on a DVD.
However, as the Library of Congress and others have observed, dye-based DVD media typically have a short data retention life even at current feature sizes, much larger than 9nm. So although the capacity of the DVDs the team envisages is impressive, they aren't likely to be much use for digital preservation.